Megilat [the Scroll of] Ruth was recorded by the prophet Samuel. …This scroll contains no reference to pure or impure or to forbidden or permitted. Why, then, was it written? To teach you how profitable is the reward for those who perform kindness. (Midrash Ruth Raba 2:15)

It is appropriate to read the Book of Ruth on Shavuot for two reasons: First, because it gives us a picture of how the poor were treated in the harvest season with sympathy and love. Secondly, because Shavuot is the yahrzeit (anniversary of the death) of King David, and in the Book of Ruth we are shown the origin of the House of David — King David was the great-grandson of Ruth and Boaz.

But a third and perhaps the main reason for our reading the Book of Ruth on this festival is because it gives us such a vivid picture of the perfect sincere convert. Shavuot is the "Time of the Giving of Our Law," and when we received it, we too, like converts, pledged ourselves to accept the Torah and fulfill its 613 commandments.

...it gives us such a vivid picture of the perfect sincere convert.

The Rabbis said [further, a fourth reason]: "Why is Ruth read on Shavuot? To teach you that the Torah is acquired only through affliction and suffering". (Yalkut Shimoni) Adds Rabbi Eliyahu Kitov [in "The Book of our Heritage"] This makes it possible to explain why we recall David through reading the story of Ruth on Shavuot: To teach you that a person can become a tool for the purpose of Heaven on this earth only through affliction and suffering. For both Ruth and David had to go through the same path of affliction.

The Sages also found hidden within its words a cryptic message that indicates the real reason why Ruth converted to Judaism. All the verses begin with the letter vav except for eight of them, scattered throughout the book. This shows that Ruth’s only desire was to bring forth a child who could be circumcised on the eight day of his life.

In his introduction to his commentary on the Book of Ruth, Rabbi Moses Alshich, one of the great sages of 16th century Tsfat [to whom even the Holy Ari and Rabbi Yosef Karo deferred in matters of scriptural interpretation — YT] says the following1 says the following:

"How can a man be sure that he is treading the right path in life, that he is walking in the way G‑d wishes him to go? How can he know if he is fulfilling G‑d’s will and will merit the Heavenly bliss that is reserved for those who fear Him? He will know when he unburdens himself of any interest in money, wealth, pride and honor; when he has only one thought in mind – to serve G‑d loyally and do His bidding exclusively for the sake of Heaven. He must not worry about the consequences of his actions or about how others react.

"This is the message of Megilat Ruth. Though some of the events related in this story might appear somewhat bizarre to us, G‑d’s intention is the show how highly He regards one whose actions are carried out solely for Heaven’s sake.

"…One who studies this Megila in depth will surely come to fear the Almighty G‑d. He will no doubt be impressed by its profound teachings, and consequently he will begin to perform his deeds solely for the sake of Heaven. I see from my own experience that this book has opened my eyes, since I have gained much knowledge and insight from studying it. I sincerely hope that the Megila itself and my commentary will lead those who study it in the way of Torah. Indeed, for this reason I have called my work, 'The Eyes of Moses'."