There are so many reasons why people discriminate against each other. In light of this, let me name a few of them from the book, The Kite Runner.
From The Kite Runner, there are three major types of discrimination. First, there is racial discrimination. Baba hates all people from the Soviet Union for their occupation of Afghanistan. And it can be presumed that those from the Soviet Union were evil towards the people of Afghanistan. From this perspective, we can say that racial problems have existed as far as human civilization.
Second, there is also social and economic discrimination. For example, Hasan was from a different social class. There is a pretty strong separation between those who have and those who do not gave. Moreover, we can see that this carries over to America, when Baba treats general Taheri well, in view of his social standing.
Third, there is some discrimination among gender. Women are treated with less respect than men in some ways. For example, Soraya is treated differently because she had a intimate relationship before marriage.