I am going to address the social and economic forces that are present in the novel and how these affect the characters. Two settings will be focused on: Afghanistan before Baba and Amir flee, and the United States, after Baba and Amir emigrate.
The social and economic forces present in Afghanistan in the beginning of the book are intertwined, as they so often are, and we usually refer to these as socioeconomic forces. We have two contrasting sets of characters, Baba and Amir, and Ali and Hassan. Baba and Amir are from the ethnic group known as Pashtun, while Ali and Hassan are from the ethnic group known as Hazara. Amir relates in the second chapter that the Pashtun have a history of persecuting the Hazara, having engaged in what was almost tantamount to genocide in the nineteenth century. This persecution arises out of the fact that the Pashtun are Sunni Muslims, while the Hazara are Shiite Muslims, and is also based upon the different physical appearance of the Hazara, whose origins are in the Mongolian Empire. During Amir's childhood, the Pashtun are the ruling class in Afghanistan, and the Hazara are a lower class, economically deprived, uneducated, and still persecuted.
This divide sets the stage for the beginning of the story, with Baba and Amir having Ali and Hassan as their servants. Yes, Baba and Ali grew up together, as do Amir and Hassan. But the socioeconomic forces of the land get in the way of what might otherwise be true friendship. Ali notes that when Baba tells stories of his childhood with Ali, never "did Baba ever refer to Ali as his friend" (25). And while Amir and Hassan play and explore together, it is clear that Hassan is the servant, the one who has done a few hours of work before Amir rises each morning, the one who must exhibit constant respect to his "better," Amir.
It is this divide that drives the early plot to a large degree. It is Amir's "superiority" (as well as his cowardice) that prevents him from defending Hassan from the attacks of Amir's peers and that allows him to manipulate events that result in Ali and Hassan's banishment from the household. And when Baba and Amir flee, there is not all that much thought given to saving two people who were, after all, mere servants in the household.
When Baba and Amir reach the United States, the tables are turned. Baba, clearly a highly educated, wealthy, and important person in Afghanistan, is now an immigrant with few resources that will help him in this new land, a member of the lower class. He speaks English poorly and his job prospects are limited. He finds work at a gas station, and he comes home with "his nails chipped and black, his knuckles scraped, the smells of the gas station - dust, sweat, and gasoline - on his clothes" (129). He has fallen far from his gentlemanly existence in Afghanistan. When Amir meets and falls in love with Soraya, who is the daughter of an Afghani general, Baba kowtows to the general, feeling inferior now in his reduced circumstances in America. While Baba and Amir do make their way in the United States, their experiences and treatment humble them, allowing them to understand what it is like to be of the lower class, as Ali and Hassan were for them.
The Kite Runner, with its sharply contrasting socioeconomic forces and characters, provides the reader with great understanding and insight into the past and the present. How one's status, religion, and ethnicity affect one's entire life and sometimes one's death is powerful, sometimes immutable, and often tragic.