Amir believes that his and Soraya's infertility is his punishment for his betrayal of Hassan years before. In addition to the infertility affecting his relationship with Soraya, it causes Amir to question himself even more than he previously did. He tries to convince himself that it might be better that he does not have a child because if he treated his friend (Hassan) so badly, he might not be a good father. He also thinks about his own father's inability to love him the way he longed to be loved and seems to think that that might be a genetic trait that Baba passed on to him.
In the end when he conquers cowardliness and his feeling of inadequacy by rescuing/adopting Sohrab, Amir begins the process of being the best father he can be to Sohrab.