The majority of everyone's dreams are anxiety dreams and nightmares are just extreme ones of those. The more emotional difficulties you have in waking life, probably the more nightmares you are going to have.

Dreams try to "balance" our unconscious a bit. I still remember when I had bizarre dreams night after night and realized that they were allowing me to "function" by day; it's much better to have a nightmare at night than while we're trying to function during the day with others around us? :-) Problems have to be worked on, one cannot "stuff" them/feelings for real, the stuffing will come out the sides of our psyche, and one of those sides is in dreams.