It is not that Arnav has any sort of feeling for La as he has never ever showed it before. He now feels guilty for having used her as a pawn and ruining here life. He is now trying to find some consolation for his misdeeds.
Khushi always thinks about others welfare more than her own, for which she will make any number of sacrifices. By placing a boulder on her heart, she is seeking blessings for Arnav-La.