Much of The Kite Runner concerns the path that Amir takes in atoning for having betrayed his friend Hassan when they were children. Although he feels guilty from the minute he turned his back on Hassan and lied about the theft, for a long time he pushes that guilt to the back of his mind and tries to pretend his actions were not reprehensible. Late in the novel, after his father, Baba, dies and Amir returns to Afghanistan, he is finally able to admit his need to atone for this behavior and to take the necessary steps to do so.
A large part of this lengthy journey involves Amir learning the truth about his father: that Baba was also Hassan’s biological father, although the boy was raised by Ali, whom he apparently accepted was his biological father. By this time, Hassan has also died. When he finds this out from Rahim Khan, Amir realizes that Baba’s preferential treatment of Hassan while they were growing up most likely stemmed from Baba’s guilt over not claiming his other son as his own. Baba took this secret to the grave.
Once Amir begins to forgive his father, he can commit to atoning for his own transgressions. The main way he does this is by becoming a father himself, by adopting Hassan’s son—and his nephew—Sohrab and committing to be a good father to him.