On a hill north of Baba's house, there's an abandoned cemetery with a pomegranate tree. This hill is where Amir and Hassan often play together. They climb the tree, eat fruit, and often Amir reads to Hassan. So, it is a place where the boys grow close to each – they even carve their names in the tree: "Amir and Hassan, the sultans of Kabul." (Hmm...didn't we just talk about a lake by the same name?) If it sounds almost like romance – or "bromance" – that's because it is. Hosseini uses the pomegranate tree to impress upon us the closeness of Amir and Hassan's friendship.
It's all in order to break our hearts. After the rape scene, when Amir and Hassan visit their tree, things just aren't the same. Amir even tortures Hassan, pelting him with pomegranates. Amir is frustrated that Hassan won't fight back – and, in a larger sense, that Hassan won't hold Amir accountable for the rape. Amir wants Hassan to accuse him of betrayal and to rough him up. At least things would be out in the open then. As it is, Hassan remains loyal and silent, bearing the burden of the rape on his own. Thus Amir throws the pomegranates at Hassan to get him to fight back. What once was the site of intense, brotherly love is now filled with Amir's bitterness, guilt, and anger.
There's even a third visit to the tree. With Afghanistan devastated by two wars, Amir returns to Kabul to look for Sohrab, Hassan's son. He stops by the pomegranate tree only to find it shriveled and nearly dead.