FiveThirtyEight discusses results of a dream analysis study from 1950 and a new study from 2013. Apparently, the majority of dreams were "bad dreams" in both the 1950 and 2013 studies.
I do not find that to be my experience with dreams. I would say the majority of them are weird - not particularly good or bad. It's rare for me to have a good or bad dream and even more rare to have a great dream or a nightmare. It would be interesting to see the dream distribution of people that have more good dreams than bad or neutral dreams to see if there is anything in common amongst those people.