I think that there are things that cannot be explained currently and that most people dismiss as superstitions. I do think that only people on a certain wavelength or at least open to the idea can sense them, largely because if you don't believe in them, you will just dismiss what you see/feel as a figment of your imagination. Plus, I believe that most ghosts aren't sentient, rather they're just echos of what they were, reacting to stimuli with base "instinct."