I think it showed Amir what kind of friendship and loyalty he was lacking. And he was not the same way with Hassan, and he needed to make up for all the years, he wanted to treat Sorhab the same way that Hassan treated him. He needs to make up for that guilt and pain he caused Hassan and himself over the years. He needed to show Sorhab that he will be there for him, and he will do anything for him. That's why at the end he was running Sorhabs kite, to show that he will be loyal to him from now on.