Why do people self-segregate by race?

[Snowstalker Mk. II]

There are also some cultural aspects which partially deal with race. For instance, despite being Cuban, I rather dislike the fake ghetto people, who are primarily Puerto Rican (though many are just white trash--few are black). On the other hand, the stuck-up white populars at the top of the high school caste system are irritating for other reasons (most act relatively nicely to me, but they're just so damn shallow).