What complicates the plot of the novel is the way that ethnicity and religion serve to create a divide between Hassan and Amir, who otherwise are more than friends, and indeed enjoy a kind of brotherhood (rather ironically, as it transpires). To be a Pashtun is a vital part of your identity, it is suggested, because it means that you are in power and are the dominant force in a country where the Hazaras have been suppressed and the Pashtuns still hold sway. Note what Amir discovers in Chapter 2 of the novel in a book about what it means to be a Pashtun:
I read that my people, the Pashtuns, had persecuted and oppressed the Hazaras. It said the Hazaras had tried to rise against the Pashtuns in the nineteenth century, but the Pashtuns had "quelled them with unspeakable violence." The book said that my people had killed the Hazaras, driven them from their lands, burned their homes, and sold their women. The book said part of the reason Pashtuns had oppressed the Hazaras was that Pashtuns were Sunni Muslims, while Hazaras were Shi'a.
It is rather disturbing that Amir continues to say the history of genocide and "quelling" is not anything that he has learnt about himself at school, so it suggests that the Pashtuns, from their position of dominance and power, have deliberately moved to "erase" certain aspects of history, re-writing it from a Pashtun perspective. To be a Pashtun therefore shapes your identity because you are in power and you are in a position of supremacy in society, and this is something that Amir uses and abuses in his relationship with Hassan.