Nightmares are caused by ignored, mis-handled and difficult issues in your waking life. Whatever is bothering you the most right now in your life is most likely what the nightmare is commenting on. It is also possible that the nightmare is connected to something from the past that you never learned to process in a healthy manner and that you continue to carry around with you to this day. You should be able to compare the actions and emotions in the nightmare to a big, heavy issue in your waking life.