Why does the human mind jump to the worst conclusion!?

Why is it that the human mind very often jumps to the worse conclusion, for instance, a man walks in on his wife, hugging another man, and instantly jumps to the conclusion that she is cheating on him. A womans favorite earrings have gone missing and she sees a pair very similar to hers on a friend of hers, she automatically umes that they are her earrings. A teenagers game system goes missing (Nintendo DS, PSP, or some other handheld device) And a friend of his buys one of the same color and type, the teenager most often umes that it is his! I won't say my mind doesn't jump to the worst conclusion, why, 9 out of 10 times it does! but why does the human mind work this way!