Amir constantly has to test Hassan's loyalty because Amir can not comprehend anyone being that loyal. Amir is very unsure of himself and those around him. Because he has no confidence in himself and because his father is so hard on him he finds it difficult to understand the love and loyalty Hassan displays to him. Amir is very selfish and self absorbed. He admits early on in the novel that he often does things that get Hassan in trouble but he continues to do those things because he finds fun in the doing.
"Torn between affection for his friend and his need for his father's love, Amir often takes advantage of Hassan's gullibility and illiteracy. Ironically, his propensity to trick Hassan—"
After the Kite running competition Hassan runs off to get the Kite that Amir has won. When he does not return right away Amir goes to look for Hassan. He finds Hassan being raped by the older bullies and he hides. After this incident Amir withdraws from Hassan because of his guilt. He witnessed a terrible injustice and does not come forward and support his friend. He is sure that Hassan would have died trying to help him if the situation had been the other way around and he feels guilty. Their friendship dissolves in Amir's guilt.
"Assef then sodomizes Hassan as his friends hold him down. Amir has one last chance to stand up for Hassan before this terrible hurt is inflicted on him. Instead, he runs terrified at what he saw. As he runs, he convinces himself that this is the price he has to pay to win Baba's affection. He later meets Hassan and pretends he doesn’t know what happened."