I believe it is primarily the wide chasm between the two boys' social classes that presents the greatest challenge to Amir. Hassan is a poor Hazara boy--an uneducated servant from the lowest social class in Afghanistan. Baba's wealth and standing in the community are never lost on Amir, and despite his happiness when spending time with Hassan (and his own lack of any other close friends), he never rids himself of the superiority complex that accompanies him whenever he is around Hassan. Hassan's harelip and Ali's own deformity are other reminders to Amir that the father and son are not equal to Baba's robust character. There are times when Amir seems to be ashamed of being seen with Hassan, and Hassan's innocence sometimes infuriates him. However, despite these obvious differences, Amir also finds himself wondering if he can ever measure up to Hassan's own worshipful adulation of his young master.
As we know Amir and Hassan were raised up together, they love each other as brothers, after what have happened to Hassan that night with Assef, Amir's weakness and non loyalty made him become far from Hassan, Amir thought that the only salvation and redeemer for his misery is to run away from the past, to forget Hassan and what have happened that night, that's why we conclude that Amir was afraid to be Hassans true friend, after all Hassan is to good for Amir (as his friend).
Because Hasaan is such a good person, and a good friend to Amir. Baba treats Hasaan like a son, but Amir just wants Baba to himself. Amir knows Hasaan is a really good person, and he knows that he would do anything for him. He takes advantage of this. Amir uses his education, like knowing big words, to taunt Hasaan so that he feels better about himself. All in all, Amir is a bit jealous of Hasaan's good nature, bravery, and Hasaan's relationship with Baba.