Assef tells Hassan that he is nothing more than a pet to Amir. Is this true? Or are they really friends, as Hassan says?

I think there are layers of meening to this. Certainly for much of their childhood, Amir treated Hassan as a pet or an indentured friend. If we dig a little deeper, we discover that Hassan meansa lot more to Amir than he knows. This is illustrated by the latter part of Amir's life when he feels the pain of guilt and regret over the way he treated Hassan. Much of Amir's life is defined by the way he treated Hassan as well as Amir's partial redemption by the adoption of Hassan's son Sohrab.