I have modified the question as I believe that is what is being referrred to in the scene on p. 36.
Assef, who we find later in the novel is more than just a bully but a rather evil psychotic, is venting his disguist about Hazaras to Amir and Hussan. In his warped thinking, Amir and his father are Pashtuns who create a problem for Afghanistan by supporting Hazara people. He poses the question to Amir, "How can you call him your friend?" (p.36).
Amir's answer is not given verbally to Assef, but as the first person narrator he reveals the answer in his thinking. "But he's not my friend! I almost blurted. He's my servant!"(p.36). This tells us much about the major flaw in Amir's character. In his own way he is highly prejudiced against Hassan despite Hassan's intense loyalty. Amir really sees him only as his servant, and also harbours a great deal of jealousy towards him because of the attention his father is prepared to give Hassan. Amir suddenly realises the horrible truth of his feelings in the intense pressure situation of being bullied and appears to be appalled by it, "Had I really thought that? Of course I hadn't. I hadn't."(p.36).