Many people have different theories about fear, but I find that the kind of universal "umbrella" theory about fear is that we are afraid of the unknown. I believe that this is true, as it applies to people who are scared of something like the spirit world - we can't really know until we die and, possibly, become part of the afterlife... thus, people are afraid because they don't know.
Or, like me, people are afraid of having no control. This is something that I work on continuously through my meditation and reflection - not being afraid that I will lose control of... well, anything!
I now see it like this: We allow fear to have it's way with us. We allow fear to become debilitating, and we allow ourselves to become terrified. While some situations are admittedly terrifying, we can learn to take control and reel ourselves in. I'm not saying to say anything like, "Ghosts don't exist, ghost don't exist," but reciting a mantra of sorts to help us through: "I am in control of this situation. I am in control of my fear. I am scared, but I will not let it overtake my focus."
Fear manifests from a deeper issue, kind of like my fear that zombies are actually real and are going to attack me. Totally silly, right? Totally! But, even though that I know this, I sometimes still have a difficult time getting to sleep since I feel the need to keep one eye open in case something decides to start snacking on my ankle. However, upon reflection, I realized that I know that zombies aren't real... but my underlying fear is that the world will end and that I will not be able to live and achieve what I want to achieve; live what I so want to live! I don't have control whether or not the world ends, but here I am, losing sleep over it. So, my solution is to live each day to the best of my ability, so that when I go to bed at night, I'm satisfied about my day and I can say that (even in the one in a trillion chance that the dead do rise) I am satisfied. Thus, I have ended my desire and this has ended my suffering. I have not mastered this and I don't expect that I will completely maybe for the rest of my life... but I am learning constantly to let go, end my desire and suffering, and not allow fear to overtake me. Don't take this as me saying, "You must have no fear!" Okay, people, this isn't Jedi training camp where I'm going to tell you that you must not have any emotions; sometimes it's fun to have fear and scare yourself! I'm saying that we should make a real effort to not let fear define us.
"Do not dwell in the past, do not dream of the future, concentrate the mind on the present moment."
- The Buddha