When it happens to me, I feel like I'm being violated. It's like I'm instinctively anticipating physical or psychological pain. It's a very threatening, upsetting feeling. From what I've been told by family members, I've always reacted negatively- even as an infant I didn't like people touching me.
I can handle hugs to a certain extent. I need some kind of indication that I'm going to be hugged, so that I can prepare for it, and when I'm done hugging- I'm done! Furthermore there are many times when I just don't want to be hugged at all. I'm just not in the mood for it.
When I worked people were constantly tapping me on the shoulder and putting their hands on me. It drove me positively bonkers. There were a couple co-workers who would randomly (playfully) grab me by the shoulders, or pat my back... I think they just thought I was extremely shy and were trying to get me to relax and loosen up.
I've found the best way to try and explain this to people is to just say, hey, I need my personal space. However, one time at a different job there was this one particularly touchy-feely woman who just wasn't taking a hint, so I yelled "DON'T TOUCH ME! IT HURTS ME!" at her. She seemed pretty offended.
I don't know. People just don't get it. It's hard to explain things to people when they haven't experienced it for themselves. Mostly I just deal with it and try to remind myself that they're just trying to be nice- no need for me to bark at people like a wounded animal.