Reading the Megillah

A few reasons why we read Rus on Shavuos:

1)Yalkut Shimoni: the Megillah describes people suffering and enduring difficulty and displaying great self sacrifice. This teaches that one can acquire Torah only through much toil and self sacrifice.

2)Chidah: Rus demonstrates the punishment for selfishness and the importance of kindness. This teaches us that one can only accept the Torah if he does kind acts. A selfish person cannot learn truly learn Torah.

3)Likutei Halahchos: Rus was the ancestor of Dovid Hamelech. He was the paradigm of tefilah and humility, two absolute essentials without which one cannot learn Torah.