"People fear what they can't understand and hate what they can't conquer." - Andrew Smith
For centuries, man kind has been afraid of what they can't explain. What is different from our "norm" suddenly becomes the most hated thing to us. That goes for both people with "abnormalities" and things that can't be explained, i.e the paranormal. Even if you don't believe in things like ghosts and demons, the fact of the matter is that the idea of FEAR against the un-realistic has become the most realistic thing to us.
Fear in general is such a strong feeling. The fear of heights, spiders, snakes, drowning, etc, all give a bigger impact in our lives than most things. My theory is that its part of human nature to try and stay alive. And it is because of this nature that we get so scared. During the time of cavemen and dinosaurs, they most likely feared being eaten 24/7 by all the things hunting them during the time of their reality. Sure, in 2014 we have no issues being afraid of going out and suddenly getting eaten by a tiger or a giant rat. But, at the same time, it would still freak us out if a tiger were to be set free from the zoo.
So, seeing as we still fear what we usually see, the fear of what we can't see just makes a bigger impact on us and our state of mind. Based of what Andrew Smith said, "People fear what they can't understand," a sort of light comes into the situation. Most people who don't believe in ghosts, big foot, and even God, say it is because they can't see it/or no prove that it exists. But, just cause you can't see it, doesn't necessarily mean its not there. And lets say, for arguments, sake that these things do exists. You still can't see them, so how would you understand it? How would you know how to defend yourself if said thing were to one day appear in front of you? Your brain can't think up a real answer, thus causing you to worry. This worry becomes fear and the fear becomes your reality.