Jumping out of one's skin?

You know the act of being given a unexpected fright and subsequently flinching?  Well, for some unfathomable reason, it never happens to me.  Jump out at me in the dark, or from behind a door, and I don't startle.  Sudden shocking effects in a movie never makes me jump.  Why is that?  I'd love to know the reason.  The only other person I've seen who is the same (Carl Morris, the pool player) is profoundly deaf.