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Just before Assef violates Hassan, he tells him (Hassan) the nature of his relationship with Amir,
"But before you sacrifice yourself for him, think about this: Would he do the same for you? Have you ever wondered why he never includes you in games when he has guests? Why he only plays with you when no one else is around? I'll tell you why, Hazara. Because to him, you're nothing but an ugly pet."
Although Assef is an evil sociopath, there is some truth to what he says.