This is a good question because Hasan is incredibly loyal to Amir; yet Amir is not able to reciprocate. This fact alone requires interpretation. Amir is afraid to be a true friend to Hasan for two reason.
First, there is the issue of social class. In other words, we need to look at their social dynamic from the context of a Middle Eastern culture where there are servants and masters. Hasan is from a different social class. His father is a successful business man with wealth, social prestige, and means. So, Amir goes to school, is educated, and has prerogatives. Hasan, on the other hand, does not have these things. He is an ethnic Hazara (lower class). So, society divides them.
Second and more importantly, Amir feels insecure. Even though he cannot put his finger on it, he feels that Hasan is more like his father than he is. Hasan is loyal, tough, and most of all brave. Here is a quote from Baba:
“A boy who won’t stand up for himself becomes a man who can’t stand up to anything.”
The one who lives up to these words is Hasan and Baba himself. The one who cannot live up to these words is Amir. So, Amir feels threatened. He feels that his father might love Hasan more. So, his insecurity does not allow him to be a good friend to Hasan. In other words, there is a psychological hinderance.