Why humans have nightmares?

Humans have nightmares because they don't have a teddy. I always sleep with my teddy, and last night I didn't, and I had a nightmare !

^ ha ha Jess

but to answer this question properly, most humans have nightmares because they are worried, anxious, paranoid or scared of something. Or, it could be that they have a special item, such as a teddy or a pillow that was passed down or given to them that they don't have with them when the fall asleep. So Jess, you were sort of right after all!