Well, there are plenty of themes that you could explore to answer this question, but one of the central ones concerns the narrator's own struggle and journey towards establishing his own identity and raison d'etre in life. Throughout his childhood, it is clear that Amir finds it difficult to know who he his and how he fits in to life. He feels constantly judged by his father's bravery and strength, and how he is able to stand up for what he knows to be right. Amir is presented as a character who is weak compared to his father. He lets Hassan be raped and does nothing and he does not stand up to protect an Afghan woman from being raped as he is smuggled out of Pakistan. Note how in Chapter Ten when Amir tries to stop Baba from defending this woman he is met with disdain:
"Baba, sit down please," I said, tugging at his sleeve. "I think he really means to shoot you."
Baba slapped my hand away. "Haven't I taught you anything?" he snapped.
Amir's own lack of moral uprightness and cowardice is constantly judged by the example of his father. It is only when Amir is confronted with the truth of his relations with Hassan that he finds the courage borne out of necessity to return to a devasatated Afghanistan and face his childhood nemesis, Assef, in order to rescue Hassan's son, and redeem himself, finding out who he is and seeing that he is his father's son after all. Thus the theme of discovering identity is shown through the journey--in every sense of the word--that Amir makes during the course of the novel.