Can u gimme one reason why someone would be a racist or a bigot ?

There can be many reasons why someone would be a racist or a bigot. My reason would be that some people are just taught to think a certain way. And they are not always exposed to the worlds diversity in a way that they can relate or come to understand. So they sort of stay in a mental rut, if you know what I mean. Like their brain cannot evolve past their way of thinking.
What do you say. What would make someone a racist or bigot.