Hassan's favorite book growing up is the Shahnama, which is the tenth-century Persian epic. Of the many tales in the Shahnama, Hassan's favorite story is that of "Rostam and Sohrab." In the story, Rostam mercilessly kills his nemesis, Sohrab, only to discover that Sohrab is his long-lost son. Sohrab's final words explain his tragic plight to gain his father's affection and Rostam proceeds to tear his clothes and cover his head with ashes when Sohrab dies. Hassan continually begs Amir to read him the story and always cry at its ending.
The story of "Rostam and Sohrab" can be metaphorically applied to Amir and Hassan's relationship. Similar to Sohrab, Hassan desires to attain Amir's love and affection in vain. Although Amir does not directly kill Hassan, he does harm him by refusing to intervene while Assef is raping him. In addition to witnessing Hassan's rape, Amir plants his gifts underneath Hassan's pillow to make it seem like he stole them, which leads to Hassan's exile.
Similar to the way Rostom was unaware that Sohrab was his son, Amir discovers that Hassan was his half-brother after he meets with Rahim Khan as an adult. Essentially, the ancient can be viewed as a metaphor for Amir and Hassan's turbulent, tragic relationship. Sohrab symbolically represents the innocent, meek Hassan while Amir represents the vengeful Rostam. Tragically, Amir traumatized and harmed his half-brother Hassan, who only desired his affection and love.