I don't have the mental energy to give everyone I come across the benefit of the doubt. When it's all said and done, the people who don't take the time to try and understand me get dismissed pretty quickly.
I know, I know, that makes me a terrible person. But it's also true that some people are more considerate than others, and all things being equal, if I'm going to expend my limited energy on someone, it's going to be the person who is a little more considerate.
I guess that's my failing, but whatever.