I think it's called the "exaggerated startle response" and is part of PTSD... like if someone taps me on the shoulder and I jump 10 feet in the air. I think that's part of it anyway - your nervous system being on constant "alert" against danger. Start by telling friends close to you that you really don't like being grabbed by surprise, and ask that they just respect that 'quirk'.

The problem is... the human body needs touch to survive. And by avoiding all the bad touches, we miss out on a lot of the good ones too (innocent hugs or pats on the back).