Horror Movies. Why?

I have never been a fan of horror movies. But I have recently been watching them with my girlfriend since she is addicted. I find myself asking why movies like “The Devil Inside” or “Paranormal Activity” and its sequels make it so big. After asking friends I found that no one would give me a straight answer. It seems like an addiction that has grown over the years and I want to know why. What is the attraction to you? Could the interest have a deeper psychological motive?