Why does Amir resent Hassan in The Kite Runner?

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.