When Amir and Hassan were children, they used to read and act out stories, and Hassan's most beloved was the 10th century Persian epic, the Shahnamah. Their favorite part of the book was that of "Rostam and Sohrab." It tells the story of the great warrior, Rostam, who slays his long-time opponent, Sohrab, in battle. As Sohrab lays dying, he reveals that he is actually Rostam's long-lost son.
The author symbolically weaves this story into the plot: Hassan represents Sohrab, and Amir is Rostam. Like Rostam and Sohrab, Hassan and Amir are unknowlingly related; and, in Amir's mind--and in his nightmares--it is Amir who causes the death of Hassan at the hands of the Taliban. Hassan names his son Sohrab because he loves the story (without knowing that he is in any way related to Amir), and author Khaled Hosseini uses it as an ironic and symbolic twist in The Kite Runner.