system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 : what causes people to have nightmares

I personally have a lot of nightmares. Very frequent. 
So what caused people to have them and why do some have them more frequently than others?
gpt
It is hypothesized that dreams are a way of preparing you for dangerous and/or scary situations. Frequency and intensity of nightmares correlates with levels of stress. We all dream each night, but you will be more likely to remember them if you wake up during the dream.

Also - parasomnias (sleep disorders)