Her purity and innocence as a child. Sesshomaru had lived his life thinking that only powerful creatures deserved to live. But after meeting Rin, he realized that just because a creature is weak, it doesn't mean its life has no value. That's why he always protects her: she has just as much a right to live as he does, no matter how much stronger he is compared to her. Learning this fact also helped him understand his father more, so he was grateful for, and to, Rin for that, too.