Those reasons include:
- Past wrongs unresolved or unforgiven.
- Feelings of superiority.
- Perceptions of inadequacy.
- Perceived lack of intelligence, import, status, social prowess in the other.
- Overt differences - in skin color, in gender, in language, in values, in education, in political views, in physical appearance,...
Seems to me that prejudice always springs from the self-centric viewpoint. When it's all about me/I, then the only way to make me/I "feel" better is to take a lesser view of others.
We can do better, I think. And we should, for the sake of our children.