Perhaps it's the fact that your head can generate anything from the past into a horrific experience. The fact that it seems so real, makes it all the more alarming. A dream feels like the real thing because all of our sensory feed has been falsified by the brain (taste, touch, smell, hearing, seeing). That's pretty much the inspiration for The Matrix:
"Did you ever have a dream, Neo, that you were so sure was real?" - Morpheus.
I had a dream a couple nights ago where I was running down a wooden staircase and there was this bug the size of a dinner plate that had glossy, dark-brown, thick, sharp legs along with a yellow-black striped large abdomen and a couple of strange tiny tentacles coming from its mouth where I presumed it hid teeth of some sort.
I tried to block it off with a couple of boxes, but it successfully came up onto my shoulder. The strange thing was, it felt real! I could feel the sensation of it crawling on my shoulder about to bite me. I woke up after that, looking over myself and brushing my shoulder because it felt so real.