When you are a part of a group, it helps to give you a sense of identity. Group associations or labels are vital to an individual's sense of identity. So you will generally believe that your group is good. But this belief has consequences. This has powerful ramifications.
Race is a socially constructed concept.
Thus, depending upon biology and culture, you are born into a certain racial group. In many cultures, your race is vital to who you are. Wanting to feel good about yourself, you feel good about your race. Feeling good about your race, feeling pride in it, you must feel that it is better than other races, at least in some fashion. And these feelings of superiority can result in disdain and discrimination if your race happens to be of high power and status in the culture, or anger and outrage if your race is of low power and status.