Recurring Nightmare

How is it possible that a recurring nightmare can cause such intense fear every time I have it? It doesn’t seem to be connected to any special date, memory or trauma but out of nowhere I have it and it’s the same each time. Same people, same words, same progression, same everything and yet I wake up terrified. WHY?

And another question, why are there so many different ones? Some I’ve had since I was a child. I don’t understand.