Although they grow up as brothers, the classist rift between Amir and Hassan is never truly filled. Amir is the son of a wealthy family, while Hassan is the son of an underclass, whose father acts as a servant to Amir's family. For a long time, Amir does not understand the difference between the classes, and does not understand why any "class" of person should be marginalized; however, his friendship with Hassan slowly begins to reflect that classist attitude, almost unconsciously. Even as a child, Amir is burdened by his conscience, but he is also a product of his environment, and he discovers that he is more likely to reflect the classist -- and racist -- attitudes of the time than he likes to admit.
But he's not my friend! I almost blurted. He's my servant! Had I really thought that? Of course I hadn't. I hadn't. I treated Hassan well, just like a friend, better even, more like a brother. But if so, then why, when Baba’s friends came to visit with their kids, didn't I ever include Hassan in our games? Why did I play with Hassan only when no one else was around?
(Hosseini, The Kite Runner, Google Books)
This distinction between the classes creates much of the tension in the early chapters, as Amir starts to come to terms with his friend being part of a disadvantaged class of people; something that would never occur to him by itself is that he is "tainted" by his friendship with Hassan, and seen as a lesser person by the bigoted children around him. The recognition of this shame eventually keeps Amir from helping when Hassan needs him the most, and Amir never forgets his betrayal.