Why do we Fear?

Why do we fear? Is it something that attacks us in the middle of the night when we are children? Does it come when we are teenagers, or in our twenties? Why does it come when we least expect it, on a bridge, in a tunnel, or atop a high building? Why do some fear being alone, or fear going outside? Why do some experience fear, while others have no clue of it? Is it something carried over from our parents, like a DNA?