The רמ"א in סימן תצ' of Shulchan Aruch Orach Chaim, quotes the words of the Avudraham. The Avudraham says that we have the custom to read the story of Ruth on the holiday of Shavuos. The obvious question is- why? Why read the story of Ruth specifically on Shavuos? What is the connection? There are several reasons offered to explain this custom. But perhaps more important are the valuable lessons that we can uncover by closely analyzing the various explanations of how Megillas Ruth relates to the holiday of Shavuos.
The Midrash Yalkut Shimoni [number 596] says the following: “What does the book of Ruth have to do with Shavuos? This [i.e. reading it on Shavuos] is to teach us that the Torah was only given through suffering and poverty.” At first glance, the Midrash’s message seems shockingly extreme. However, after reflection, maybe the message of the Yalkut is as follows. By reading the book of Ruth, we attempt to correct a possible misconception that we might have about the nature of being Hashem’s chosen people. The background of the holiday of Shavuos is the holiday of Pesach. When the Jews left Mitzrayim, Hashem performed numerous miracles on their behalf to rescue them from slavery in Egypt. Hashem then took them into the Sinai desert and performed more miracles to show his love and concern for the Jews. For example, Hashem gave us the “Clouds of Glory” which protected the Jews from all types of difficult weather conditions. According to some commentators the “Clouds of Glory” even functioned as a transportation vehicle moving the Jews from one location to another without them having to tire themselves by walking. Hashem provided miraculous bread for the Jews to eat in the desert called the מן. Our sages tell us that the מןtasted like any flavor the person who was eating it desired. Finally, Hashem brought the Jews to Mount Sinai and gave them the Torah accompanied by a demonstration of yet more incredible miracles. After all these wondrous events leading up to Matan Torah, one could have logically made an assumption. So long as we observe the Torah and the mitzvos --- our lives will be comfortable all the time and all our physical needs will be taken care of by Hashem just as they were in the desert. In order to dispel this incorrect belief we read the story of Ruth on Shavuos. We are supposed to understand by reading this story that just like Ruth had to endure much suffering and years of poverty until she was finally accepted as a Jew by her peers, so too all of us may endure tests from Hashem throughout life- even if we are meticulously observing the commandments of the Torah. This is a basic lesson about the life of a Jew, and we are taught it by the story of Ruth.
The story of Ruth begins with the famine that ravaged Eretz Yisrael causing the leader of the generation, Elimelech, and his pious sons Machlon and Chilyon to leave Eretz Yisrael and move to the land of Moab. The Gemara in Maseches Babba Basra (91a) records the following comment of Rabbi Shimon Bar Yochai “Elimelech, Machlon and Chilyon were the greatest men, and the leaders, of their generation. This being the case, why were they punished (that they all died)? Because they left Eretz Yisrael to go to chutz la’aretz.” The gemara goes on to cite the opinion of Rabbi Yehoshua Ben Korcha who argues with Rabbi Shimon Bar Yochai. Rabbi Yehoshua Ben Korcha says that Elimelech and his family were justified in leaving Eretz Yisrael due to the famine. The reason they were punished he explains is because “they should have beseeched Hashem to have mercy on his people and they did not.” The question that needs to be asked is, why in fact did they not pray to Hashem to end the famine and have pity on His people?
From the words of the Yad Ramah (יד רמה ), in his commentary on this gemara, we can infer an answer to this question. The Yad Ramah says"שמעינן מינה דמאן דחזי למבעה רחמי על בני דורו ולא בעי, מיענש" . The Yad Ramah states that only a person who is worthy to ask Hashem for compassion (such either Elimelech, Machlon, or Chilyon) for his generation is punished for not doing so. It would seem that this principle regarding תפילה during a צרה is inconsistent with what we find in חז"ל . In מסכת תענית it states that when there is a צרה, all of Klal Yisroel must daven for a ישועה. Given that, how can the יד רמהsay that only select people are “worthy to ask Hashem for"רחמנות (and are therefore punished for failure to do so)?
Perhaps the יד רמה analyzed words of Rabbi Yehoshua Ben Korcha as follows. As noted above, according to Rabbi Yehoshua Ben Korcha it needs to be understood why indeed didn’t אלימלך, מחלון וכליון daven for the famine to end? The יד רמה means to answer this question by assuming that in fact אלימלך מחלון וכליון did, like the rest of כלל ישראל, daven to Hashem to end the famine. However, from the fact that they were nevertheless punished, the יד רמה deduces that certain people- צדיקים - have an additional responsibility beyond that of a regular Jew. Apparently צדיקים must daven either longer, more frequently, or with a greater intensity than an average Jew is required to. This enhanced form of תפילה carries with it an opportunity cost since it takes away from the צדיק’s regular routine. Nevertheless, one who is “worthy” to engage in such תפילה, is required to do so. Rabbi Yehoshua Ben Korcha is implying that owing to their great humilityאלימלך מחלון וכליון did not view themselves as being “worthy” of this enhanced role of begging Hashem for mercy. The truth was, that Elimelech, Machlon, and Chilyon were “worthy” and thus obligated to engage in this form of prayer. Since theyfailed to fulfill this obligation, they were punished severely. [The דוחק in the aforementioned interpretation of the יד רמה is that his לשון stresses the עונש as being the חידוש of Rabbi Yehoshua Ben Korcha. According to the פשט suggested above, the main חידוש is the fact that people who are singularly “worthy” must daven more than regular Jews. The יד רמה’s לשון does not stress this point.]
Based on the aforementioned understanding of the Yad Ramah, it comes out that Elimelech, Machlon and Chilyon had a misplaced sense of humility. They viewed themselves as unworthy of asking Hashem for compassion on the nation. However, too much humility can lead to a person failing to perform his obligations to the klal. Perhaps this is what occurred with Elimelech Machlon and Chilyon.
The story told in the above gemara is very relevant to Shavuos. We are all familiar with the Midrash about how Hashem looked for a mountain on which to give the Torah, and finally chose Mount Sinai. Mount Sinai was selected because of the fact that it was the smallest and therefore the most “humble mountain”. Hashem linked the giving of the Torah to a symbol of humility and modesty- in order to show us that only one who is truly humble can acquire Torah. However, when reflecting on this message during Shavuos, we are somewhat at danger of taking the important trait of humility too far, as Elimelech and his sons did. In order to help us put the value of modesty in its proper proportion we read the story of Ruth and learn about the dire consequences of too much modesty. We see this from the tragedy that befell Elimelech, Machlon and Chilyon as explained above. Thus we emerge from Shavuos with an accurate, balanced notion of the value of humility in Torah thought. 
A different reason for reading the book of Ruth on Shavuos is offered by the Elya Rabba (ibid siman 490). He refers us to a Midrash (also a gemara in Sotah 14a) which states “Torah begins with chesed and ends with chesed”. The book of Ruth is a story about the importance of chesed. (Chazal tell us that Boaz, who Ruth marries at the end of the megilla, was much older than Ruth. Indeed he died shortly after their marriage. Nevertheless Ruth was willing to remain single until circumstances allowed her to marry Boaz and have a child. The Ramban explains in Parshas Vayeishev that the union between Ruth and Boaz was a form of “Yibum.” When a wife’s husband dies without children it is a big chesed for the soul of the deceased brother if his surviving brother or any other relative marries his widow. Because Ruth’s husband Machlon had died without children, Ruth desired to benefit his soul by marrying his cousin Boaz in a yibum-like mitzvah. However in order to accomplish this Ruth had to endure a long period of time as a widow. Also as noted above, Boaz was much older than her. Therefore Ruth’s willingness to perform this mitzvah was a selfless act of chesed. Indeed Boaz said to Ruth in perek 3 pasuk 10 “Blessed are you my daughter, your act of chesed is great that you choose not to look for a younger husband.”) Indeed the midrash (Rus Raba 2:13) goes so far as to say אמר ר' זעירא—מגילה זו אין בה לא טומאה ולא טהרה ולא איסור ולא היתר. ולמה נכתבה? ללמדך שכר טוב לגומלי חסדים. “This megilla contains no halachos. So why was it written? To teach us the great reward that will come to those who act kindly”. By reading this story on Shavuos, the day of Matan Torah, we highlight the idea that Torah “begins with chesed and ends with chesed”. This is a nice idea but it requires some elucidation. What does it mean that the Torah “begins and ends with chesed?” Two explanations follow.
The Maharal (Chidushei Aggados Sotah 14a) explains that despite the many stringencies and discussion of punishment that we find in the Chumash we should not form an impression that the Torah was designed by Hashem to hurt us (chas veshalom). Even when Hashem punishes the Jews for their sins, Hashem’s intention is only to cleanse us of our sins so that we can merit a portion in Olam Haba. This is a tremendous chesed.
The Iyun Yaakov (ibid) gives a different explanation about the idea that “Torah begins and ends with chesed”. He says that the message for us is that studying Torah is not alone sufficient for a person to draw close to Hashem. Even a great talmid chacham must supplement his Torah study with acts of chesed. By reading the story of Ruth on Shavuos we are teaching ourselves these important values.
Another connection between Ruth and Shavuos is as follows. The Birkei Yosef (siman 494 citing the Simla Chadashah) points out that both the birthday and yahrtzait of Dovid Hamelech were on Shavuos. He explains that the book of Ruth traces the lineage of Dovid Hamelech and shows that his great grandmother entered the Jewish people with the approval of the great sage of the generation- Boaz. Even though Ruth came from the nation of Moab from which we are forbidden to marry converts, Boaz correctly interpreted this halachah as prohibiting only male converts from Moab, not female converts like Ruth.
Superficially, it would seem that according to this explanation for reading Ruth on Shavuos, we are reading the story of Ruth because of an incidental reason not having to do with the essence of what Shavuos is mainly about (i.e. Matan Torah). However, it has been suggested that there is a connection between talking about Dovid Hamelech and understanding Matan Torah. The gemara in Shabbat (88A) tells us that when the Jews accepted the Torah at Mount Sinai and declared “na’aseh ve nishma נעשה ונשמע “ Hashem rewarded every Jew with two crowns- one for na’aseh and one for nishmah. This gemara stresses the idea that the Torah is a form of malchut, royalty. By talking about the lineage of Dovid Hamelech on Shavuos we highlight the association between the Torah and royalty. As Torah observant Jews it is incumbent upon us to always keep in mind that it is adherence to the precepts of the Torah that gives us our stature as Hashem’s “Kingdom of princes” (Shemos 19:6).
The next reason offered regarding why we read Ruth on Shavout also pertains to the fact that the megilla traces the lineage of Dovid Hamelech. However, this new pshat focuses on a different aspect. As noted earlier, the Torah forbids a Moabite convert to Judaism from marrying an ordinary Jew- ‘ .לא יבא עמוני ומואבי בקהל הInstead, such a convert must marry either a fellow convert or a mamzeres. The Talmud Yerushalmi (Yevamos 8:3) tells us that in the times of Ruth there was a dispute among the sages regarding the parameters of this prohibition. Some sages held that this prohibition applies both to male and female converts from Moav. According to this opinion, Ruth was forbidden from marrying (i.e. doing Yibum with) any of her deceased husband’s relatives. The midrash (Rus Raba 7:7) says that Boaz’s uncle, Tov, subscribed to the strict opinion. Consequently, he declined to be מיבם Rus. He told Boaz -"לא אוכל לגאל לי פן אשחית נחלתי " “I cannot redeem [Ruth i.e. do Yibum] lest I destroy my legacy”. Boaz belonged to a group of sages who, utilizing methods of biblical exegesis that are delineated in the oral Torah ( תורה שבעל פה ), determined that only male Moabite converts were prohibited, not female converts like Rus. The gemara in Kesubos (7b) states that Boaz assembled ten sages to publicly announce this halachik decision, and only afterwards did he proceed to marry Ruth.
The Midrash Tanchuma in Parshat Noach (#3) relates that although the Jews were willing to accept the written Torah ( תורה שבכתב ) and declared נעשה ונשמע -they declined to accept the oral Torah ( תורה שבעל פה ). The Tanchuma (as interpreted by the Alshich) states that they were afraid that since תורה שבעל פה is so difficult to study and comprehend, they would frequently violate its precepts inadvertently. The Tanchuma goes on to say that Hashem forced the Jews to accept the oral Torah by suspending Mount Sinai above them and threatening them with death if they did not agree to accept תורה שבעל פה : כפה עליהם הר כגיגית ואמר להם אם אתם מקבלים את התורה מוטב ואם לאו שם תהא קבורתכם. As a result, the day of Shavuos not only reminds us that the Jews accepted the Torah, but also (unfortunately) that they had to be coerced into accepting תורה שבעל פה . In order to instill in us how crucial the oral Torah is, and demonstrate that תורה שבעל פה is inseparable from תורה שבכתב , we read the story of Ruth on Shavuos. By hearing this story we are made to understand how the union of Boaz and Ruth, which in turn led to the birth of Dovid Hamelech, and which in the future will produce Moshiach ben Dovid--- was only possible because of the clarifying light that the oral Torah shines on תורה שבכתב.
A well known connection between Ruth and Shavuos is made by the דרכי משה in סימן תצ' . He states that we read Ruth on Shavuos because אבותינו שקיבלו התורה נתגיירו וכן רות נתגיירה . The Darchei Moshe seems to be saying that by reading about Ruth the convert, we remind ourselves that we too underwent conversion on Shavuos at Mount Sinai. The question is, why is this an important thing to highlight on Shavuos?
In פרשת ואתחנן (ה':כז) the Torah recounts that after Matan Torah, ,משה רבינו proclaimed to Klal Yisroel . "שובו לכם לאהליכם" The gemara in Moed Katan (7b) explains that "אהל" is a reference to a person’s wife.משה רבינו was thus telling the Jews that now that the "שלשת ימי הגבלה" (during which the men had to separate from their wives) were over, the men could return to their wives. The Meshech Chochmah observes that it seems probable that prior to Matan Torah, many Jews had married relatives whom, post-Matan Torah, were now forbidden under the halachos of .עריות An example of this was Moshe’s own father Amram, who was married to his aunt—Yocheved. After Matan Torah, marrying one’s aunt was prohibited. All the Jews who had earlier married relatives that were now forbidden should seemingly have had to divorce their wives. This being the case asks the Meshech Chochmah, how could Moshe make a categorical statement, addressed to all of Klal Yisroel, permitting all the men to return to their wives?
The Meshech Chochmah answers that perforce, the גירות that Klal Yisroel underwent at Matan Torah rendered them new people halachicaly. All previous familial ties were considered null and void. Thus after Matan Torah a man who had earlier married his aunt was allowed to remain married to her. Owing to the גירות of Matan Torah, they were no longer considered as aunt and nephew. The Meshech Chochmah goes on to say that this is the source for the Talmudic dictum גר שנתגייר כקטן שנולד דמי “a convert is considered like a newborn child”. This means that his prior relatives are no longer considered relatives in the eyes of halacha.
With the aforementioned concept in mind, we can now suggest a deeper understanding of the דרכי משה ‘s pshat , i.e. that we read Ruth on Shavuos because “she was a convert and we too converted at Matan Torah”. The גירותof Matan Torah introduced us to the idea of גר שנתגייר כקטן שנולד דמי. Although this is a halachik concept, it has a hashkafic connotation as well. It conveys the message that Matan Torah, Shavuos, is a time of renewal and rebirth. If heretofore we were lacking in certain areas of our Torah observance, each Shavuos we should focus on the idea of גר שנתגייר כקטן שנולד דמי and be inspired to renew our commitment to the Torah. This encouraging message is communicated to us by reading Ruth on Shavuos and thereby recalling the גירות of Matan Torah.
A similarly comforting reason as to why we read Ruth on Shavuos is offered by the פני יהושע אור. In order to understand his explanation, it is necessary to first consider what ourעבודה is during the forty-nine days between Pesach and Shavuos. In משנת רבי אהרון (vol. 3 pg. 13), Rav Aharon Kotler זצ"ל states that these days areכנגד the מ"ח דברים שהתורה נקנית בהם. Inאבות פרקי (chapter 6), the mishna delineates forty eight methods that need to be employed in order to properly study and acquire the Torah. Rav Aharon says that on each day between Pesach and Shavuos we are supposed to work on one of these forty eight קנינים. On Erev Shavuos, day forty nine, we are supposed to combine all forty eight methods and achieve thereby a high level in learning.
Alternatively, during this time period, the Jews who left Egypt elevated themselves each day from the nadir of spiritual impurity into which they had sunken in Egypt. Each day after leaving Egypt, they left one of the forty nine gates of טומאה and replaced it with one of the forty nine gates of קדושה. This culminated on Shavuos, day number fifty, when the Jews entered the fiftieth gate of קדושה and then received the Torah. Nowadays, our job during this time period is to try to reenact this process of spiritual growth during the forty-nine days between Pesach and Shavuos.
With the above background in mind we can now consider the following common scenario. A Jew reaches Shavuos having failed to utilize the previous forty nine days to improve his spiritual standing. Likewise, this Jew made no effort at all to work on the מ"ח דברים שהתורה נקנית בהם. A Jew who arrives at Shavuos in such a state is likely to be extremely downtrodden and depressed. He is probably thinking: “what use is Shavuos for me when I didn’t prepare?” In order to encourage such a person, says the אור פני יהושע, we read Ruth on Shavuos. Ruth began as a gentile and an idolater. Yet despite those lowly roots, she rose to become a convert, accept the תורה, and became a part of כלל ישראל. This is meant to encourage those of us who arrive at Shavuos in a lowly state, not to give up. Ruth overcame her circumstances and accepted the Torah. Similarly, we too can reach new spiritual levels on Shavuos, notwithstanding shortfalls in our preparation during the preceding forty nine days.
From all of the above, we can distill eight discrete lessons that we are taught by reading the book of Ruth on Shavuos:
1) Strict adherence to Torah and mitzvot does not guarantee a carefree life such as the Jews’ existence in the midbar.
2) The virtue of humility must be weighed against a person’s communal obligations.
3) The punishments in the Torah, notwithstanding their severity, are intended for our benefit. As such, they are acts of chesed.
4) Torah study and erudition are insufficient to attain spiritual perfection. Involvement in chesed is also necessary.
5) The stature of the Jewish people as Hashem’s princes is a byproduct of the fact that we received the Torah.
6) The oral Torah is crucial and inseparable from the written Torah.
7) Matan Torah was a time of renewal, rebirth. Recalling that should serve as an impetus for us to renew our commitement to Torah life, and not be held back by previous lackings.
8) Even if one did not utilize the seven weeks between Pesach and Shavuos to prepare for Kabbolas HaTorah, he should not despair. Observed correctly, the day of Shavuos alone affords an opportunity to attain great spiritual heights.
 The custom of the Ger and Chabad Chasidim is not to read Ruth. ( ספר נטעי גבריאל )
 Rabbi Dovid Feinstein shlita, in his work “Kol Dodi” on Ruth, understands the point of this yalkut differently. He says that the lesson which the yalkut wishes to bring out is that limud hatorah, becoming a true talmid chochum, can only be achieved through poverty and suffering. I find this difficult to understand. As a woman, Ruth was not involved in torah studies. Her struggle was simply to become Jewish, not to become a scholar. So how can we extrapolate from Ruth’s struggle what it takes to become a talmid chochum?
 Indeed we find this type of “balancing” rationale given by the Magen Avraham ( Orach Chaim 490:8) to explain why we read Koheles on Sucos. Sucos is a time of simcha זמן שמחתינו -due to the abundance of physical pleasures available at that time of the year (i.e. the season of gathering in the harvest). In order to prevent us from getting too caught up in this gashmiyus we read koheles which talks about how the pleasures of this world are in truth הבל worthless.
Similarly, on Pesach we celebrate our emancipation from Egyptian slavery-זמן חרותינו . However this might lead us to consider our current subjugation to the nations of the world and contemplate rebelling (in the example of Egypt, Libya, Syria, and Tunisia!)! In order to preempt such ideas, which Pesach observance might conceivably inspire, we read the Song of Songsשיר השירים . This megilla contains the famous two oaths (discussed in Maseches Kesubos 111(a) that Hashem adjured the Jewish people when they went into exile: 1) not to rebel against the gentiles and 2) not to band together as a group and try to recapture Eretz Yisroel by force.
 Introduction to Otzar HaMidrashim on Rus, Freidman Edition