Fear is a survival tactic that is built into our species. You are all familiar with the fight or flight syndrome, and that is where your fear originates. In the early days of mankind, the human body's fight or flight syndrome was meant to protect you from the dangers in your environment. Today, however, your fears often center around emotions rather than physical dangers... although your body does not know how to distinguish between the two.
When you are afraid, your body functions still operate as if you are facing a physical threat, and the same fight or flight system kicks in to prepare your body to either fight or flee. The stress hormone cortisol, as well as your adrenaline, starts pumping so that you can react quickly to danger. If you are a worrier or you experience sustained fear because of your health, death, grief, divorce, loss, etc., then the fight or flight syndrome bathes your brain in cortisol without the ability to dump its immense load. As a result, sustained worry and fear can, in fact, change your brain architecture, as well as your impulse control.
On the other hand, if you face serious threats, such as poor health or if you are in a dangerous situation, then fear is an appropriate response. Further, if your fear is obsessive, and all-encompassing, you may be dealing with a conflict and compensation for a particular or earlier complex. The key is how to manage and mitigate that fear so that it doesn't paralyze you from acting on your own behalf and for your own well-being.
For example: if you are afraid of deep water, it may not be the water you fear, but an inner conflict symbolized by the water. Of course, there are real things to be afraid of: illness, war, stranger danger, and emotional trauma, etc.
The key is to know yourself -- to understand your situation and your fears. Don't be afraid to seek professional help if your fears become too large for you to manage. Professional counseling can help.