In the book The Kite Runner the idea that the past is always there is evident in the way that it manifested as guilt and insomnia in Amir, the revelation of Hassan as Amir's half-brother, and the negative reunion between Assef and Amir.
The story begins with Amir telling how he had become what he is today because of an event that had happened when he was twelve. He had tried to escape from the event, but the experience that happened to Hassan in the alley of Kabul ahs remained with him.
"Looking back now, I realize I have been peaking into that deserted alley for the last twenty-six years."
When Amir visits Rahim Kahn in his native country he learns that his own father, Baba, had a sexual encounter with the wife of his friend/servant and that Hassan was actually his half brother. His father's actions from the past have now gone to the present as well to create a situation that Amir must now face. He must rescue Hassan's child.
"I want you to go to Kabul. I want you to bring Sohrab here," he said." (220)
When Amir goes to rescue Sorhab he finds that it is his old enemy Assef who had raped Hassan and killed him that now had his son. He makes the comment reminding Amir of the past, bringing the past once again to the surface.
"We have some unfinished business, you and I," Assef said. "You remember don't you?"(286)