Amir resents Hassan because Baba shows him more attention and interest as a child. Even though Hassan suffers from discrimination because he is a Hazara and lacks many of the opportunities and privileges Amir enjoys, Amir resents Hassan for gaining Baba's affection and respect. Amir desperately wishes to gain his father's love and attention but is not naturally athletic or masculine. In contrast, Hassan is naturally gifted, fearless, and completely innocent. Amir's negative feelings of jealousy and resentment stem from his low self-esteem and lack of affection. When Hassan is present around his father, Amir feels neglected and is treated as an afterthought. There are many moments when Amir pleads with his father to not bring Hassan along with them because he wants Baba's full attention. It is Amir's jealousy and resentment that motivate him to not intervene when Assef is raping Hassan, which eventually ruins their friendship. As an adult, Amir discovers that Hassan was his half-brother, which explains Baba's fascination and admiration for him.
The issue of a father's love or attention becomes the root of why Amir resents Hassan. Amir is bookish, more cerebral, and much more introverted than Hassan, whose nature as a child is quite the opposite. As a child, Amir does not understand why his father demonstrates more outward affection to Hassan, and this becomes the basis of his resentment towards him. Of course, he later understands why this was the case. Both are still friends, and Amir does not let his father's attention ruin the fundamental relationship between both of them. Yet, this issue, or wedge, between how the father perceives both becomes the fundamental issue in reflecting the resentment that Amir has towards Hassan. It is also the reason why Amir does not stand up for nor help Hassan in his time of need. Consequently, it is the reason for the haunting that still follows Amir even when he leaves Afghanistan, triggering his need to return to it.