What causes fear in humans?

A lot of stuff cause that . The human learn than by time . Some time human fear snakes but he never saw it . Or human are fear from mouse , where in the fact mouse are not danger for us like the snake or tiger .

First of all, some psychologists state that we are born with primary anxiety (see Karen Horney for example).

But I think that you didn't ask about theoretical matters but physiological background of anxiety.

In this case, it is a temperamental matter. The more reactive you are, the more fear you experience. It is connected with activity of sympathetic system.

There are different conceptions on temperament. Read about Jerome Kagan or Jan Strelau or Eysenck, Pavlov, Hippocrates-Galen classification.