Why do we fear what we don't know?

If something bad happens we want to know. If something good happens we want to know. Either way we want to know. We fear what we don't know because we have never heard about it. Ever heard the saying don't let history repeat itself. Well it's the same concept. If we knew of the bad thing we wouldn't let it happen again. That's why we want to know everything so we don't have to fear the unknown.