Why do people self-segregate by race?

[Bacon King]

Almighty God created the races white, black, yellow, malay and red, and he placed them on separate continents. And, but for the interference with his arrangement, there would be no cause for such marriage. The fact that he separated the races shows that he did not intend for the races to mix.

Everyone, this is actually a direct quotation from opinion of the original trial judge in the Loving v. Virginia case, wherein the Supreme Court decided to overturn laws banning interracial marriage.