Nightmares and dreams reflect your perception of reality. E.G. if you are afraid of something in your real life, your body will render it in a nightmare.
Not suggesting this is the complete truth, but it partially is.

Why dream/nightmare? Your body needs to filter out all the impressions you get throughout the day (sub- and consciously). I also have nightmares, and bad dreams, and I know it sucks. But it's easier to accept it when I know why I have them.

*Hugs*