Dreaming about death every night?

A few months ago I started to get nightmares every single night and I'm still getting them. They're always reoccurring with very slight differences and are always about death. At first it was always about a family member or a friend dying, but now they're all about me dying and they're all extremely vivid and life-like. I never really remember the beginning of them or what happened that made this person want to kill me, but I remember every other detail so perfectly you'd think I was getting tested on it. Typically at the beginning I'm with a friend or my boyfriend and then I leave to go get something and then I begin getting attacked by a faceless person, I run away from them, but they won't stop chasing me. Finally they catch up to me and go to kill me, its a different way every night, but whatever they use to try to kill me I feel it like it was really happening. At this time I realize I'm asleep, but I can't wake myself up and everything gets even more real. The person teases me, but won't get along with it and kill me. I can't move or anything and I have to watch myself slowly die. Why do I keep having this dream!? What's wrong with me!? It scares me so much I dread going to sleep and I hate being alone now. What can I do to stop this?? I eat good, I get exercise, I'm overall a very happy person, I'm not depressed, I don't have anxiety, I'm not stressed out about anything, I don't watch anything even remotely scary on tv. What causes this!?