In "The Kite Runner," why do you think Amir recalls this memory when he witnesses Hassan's tragedy in the alleyway?

It is this central guilt that also keeps Amir from healing his relationship with his father, his roots, and himself.  He idolized his father with the eyes of a child.  His father tells him that the worst thing to do is to lie.  Amir feels that his father has been an honest man, unwilling to compromise even in the face of death as illustrated in the confrontation with the Soviet soldiers on their journey out of Afghanistan.  However, he keeps this secret, and later on realizes that he has been a coward not only to a friend, but to a brother, and in essence has betrayed his father by betraying his father's son.  It is necessary as a literary vehicle, because it becomes the impetus and driving force for his return to Afghanistan and his quest to bring home his nephew, Hassan's son.  Amir holds fast to the guilt and this act of cowardice in his past in order to anchor him to his commitment of the rescue of the child from the same blue-eyed villain who assaulted his brother, Hassan.