Often ones dreams are just random, really accentuating the point that anything can happen.

Often people dream of doing something bad. This is a form of nightmare where you are the bad guy. Usually these dreams are accompanied by a feeling of guilt or dread.

Nightmares can include all sorts of random events. A repeating childhood nightmare I kept having was where I was being chased by hands trying to pull me away. I discussed it with a few different people, and realized that it is in fact, a rather common form of nightmare.