How come nightmares have all the things you are scared of in them?

Sometimes these are the fears we carry mixed in with the events of the day..somethings upset us throughout out the day and if we do not deal with them when the transpire and put an emotion to them they tend to come up in our dreams and they can become nightmarish or even angelical..depending on how we collect and store information of the days events in our own minds.

Your mind has that terrifying ability to wander.

Same as fear, to overcome it. Once you do research on what kind of fear/nightmares bugs you, it will go away.