This is a great question. The title is about the most important incident in Amir's life. He witnesses the rape of Hassan as Hassan was going to look for the second place kite in a kite tournament. Amir sees the action, but he does not do anything to help his faithful friend.
This cowardice of Amir shapes his life. And he can never forget it. He feels an overwhelming sense of guilt, because he knows that Hassan would die for him. Eventually he falsely accuses Hassan and he is sent away. This, too, defines him in a negative way.
As an adult, Amir is able to redeem himself. He finds out that Hassan had a son and that Hassan was killed by the Taliban. In light of this, Amir goes to get Hassan's son and in the end adopts him.
In short, Amir never forgot his friend, the best kite runner in the world.