Wow... big question. If you look at the loc.gov link (you have to scroll down to a part that starts on p. 7 out of the 20), you'll see a list of three reasons why stereotypes form. Let's look at them:
- Stereotypes are aids to explaining the world. So there has to be some issue to explain -- one that the stereotype will help. For example, in Maycomb, a stereotype might be used to explain why blacks are poorer and less educated. So one thing that is necessary is some difference that needs to be explained.
- Stereotypes are energy saving devices. If we just believe in stereotypes, we don't have to think too hard about what is really causing the thing that needs to be explained. So it is necessary for people to be A) lazy enough that they don't care about getting the right answer or B) uninterested -- they don't really care about the right answer. This means that the difference that needs explaining can't be too important. In Maycomb, for example, the whites may not really care why the blacks are poorer and less educated.