The answer to this question can be found in Chapter 13 of this novel. This is when Amir and Soraya try to have children and then have tests to establish if there are any problems. Although nothing can be detected, they are still unable to conceive. Soraya is of course trememndously disappointed and upset. However, Amir, because of who he is and what he did in his childhood, feels that not being able to have children is some form of punishment that has been inflicted upon him for not standing up to Assef and saving Hassan. Note what he says:
...perhaps something, someone, somewhere, had decided to deny me fatherhood for the things I had done. Maybe this was my punishment, and perhaps justly so. It wasn't meant to be, Khala Jamila had said. Or, maybe, it was meant not to be.
The interesting change in emphasis based on the slight change in word order reveals that Amir feels that not being able to have children is a result of his actions in his childhood. He still lives with massive, crushing guilt, and this is something that he struggles with daily. Note the way that the "unspecified" problems causing infertility support this conclusion.