This question gets to the heart of the novel.
Amir's best friend is actually Hassan. However, their relationship is complex owing to Amir's relationship with his father, Baba.
Baba is a distant man. Without a doubt he loves Amir and shows him kindness and faithfulness. But Amir is unlike Baba. Baba is brave and courageous, whereas Amir has a difficult time standing up for himself or for what is right. For this reason, Amir feels disconnected to his father. I should also say that it is not entirely Amir's fault. Baba has Hassan out of wedlock and he is not able to acknowledge him as his son owing to social pressures.
On the other hand, Hassan is brave and sacrificial. So, when it is clear that Baba also cares for Hassan, Amir becomes jealous of Hassan. For this reason, he has a hard time loving him as a friend. Amir at one point watches Hassan get raped and does nothing.
What this tells us about Amir is that he is a selfish coward who goes about seeking love in the wrong way. Later on in life, Amir also realizes this and seeks redemption. He fends for and adopts Hassan's son. Amir emerges as a brave man who sacrifices himself to do what is right.