It is many fears combined. Top among them are...
When you get a cut, or if you lose a finger, there is usually some pain involved. Same said when you have an operation at the hospital. The body is built with a natural aversion to pain and death.
People who know that they are dying, especially if it happens quickly, like in a car accident where you have only minutes of life left, become very realistic. They put aside their foolish ideals of life, including the fact that God does not exist. They fear having to meet Him, because they know instinctively that they have not obeyed His laws during their lives.
I take it from your comments BADecker that you've been present when many people have died, holding their hand and comforting them in their dying moments. May I ask what line of work you did this in? Or is this simply an opinion based on your own beliefs?
[tongue in cheek] The thing that I fear most in death, is that BADdecker is right and there is a God. I'll have to put up with his religious nonsense for the rest of eternity....yes, that's right, he bet on the wrong God
[/tongue in cheek]