Chazal ask: Megilas Rus does not teach us issur v’heter, nor tumah v’taharah; why was it written? The Midrash answers that the megillah was written teaches us the reward for gemilus chessed.
(Parenthetically, most people focus on the punchline of the Midrash, but what strikes me is the assumption of the hava amina that the primary purpose of megilos, and I assume other sifrei Tanach, is to teach is halachos. Are these issues -- issur v’heter or tumah v’taharah – what you thought about when reading Megillas Esther?)
In addition to the theme of chessed that permeates the megillah (see R’ Chaim Friedlander’s Sifsei Chaim that goes on for pages enumerating examples), Rus is also the story of David haMelech’s family. Shavuos is the yahrzeit and birthday of David, and we focus on his lineage.
It’s not coincidence that these two themes – chessed and malchus – come together. The gemara (Nedarim 24) discusses a case of a person who takes a neder to force his friend give him something. “Lav malkah ana” – the neder makes a statement that “I’m not a king who gives things to you and gets nothing in return.” Being a king, malchus, is about giving to others. The ultimate gomeil chessed, the person who gives and takes nothing in return, is living the life of a king.