It could be a trauma thing: I have a buddy who thinks dreams in general are terrifying after having a lot of nightmares. Maybe you are the same, but not as aware of it.

You haven't given us a lot to go on. What is your relationship to dreaming? What are your thoughts on the nature of dreams? Have you had a lot of nightmares? Are you a generally anxious person? Could it be that you are afraid that your waking body is unprotected and unconscious?