Why is Sohrab's smile lopsided when he finally responds to Amir?

Sohrab is emotionally damaged. He has been from orphanage to orphanage. He has lost his parents most brutally and harbors resentment against Amir. Finally when Amir gets to America he refuses to talk or even look Soraya (Amir's wife). It is only when Amir begins to reminisce about Hassan and show Sohrab some kite tricks that he begins to finally soften hence the "lopsided smile."