Baba has a very close relationship with Hassan; he treats him like he's his own son. There's a good reason for this: he is his son. Amir doesn't find out about this until later on, but when he does, it explains everything. Growing up, Amir was always jealous of the attention that his father lavished on Hassan. So when Hassan was raped by Assef, he seized the opportunity to have him sent away. This way, Amir hoped to take what he believed to be his rightful place in Baba's affections.
After Amir discovers the truth about Hassan's parentage, he realizes that he has to make amends for the appalling way he treated him when they were young. This involves traveling back to Afghanistan to adopt Sohrab, Hassan's orphaned son. In symbolic terms, Amir is trying to bring Hassan back through his son, reestablishing the connection he had broken all those years ago when he was instrumental in Hassan's banishment.