Sesshoumaru's fatherly love of Rin and Rin's innocence and loyalty has made for the sweetest part of the whole series, and this is the perfect song for it (very Rin-like voice). You certainly don't see Sesshoumaru telling Rin to sit everytime he feels mildly annoyed.
Rin and Sesshomaru are one of the only reasons I still watch this show. They are sooooooo cute together! I wonder what will happen after ep 26 of the final act? ;)