I don't know what it is, but for years now whenever my hair is pulled hard enough to hurt, it is a HUGE emotional trigger for me. I totally flip out.
I was working at a haunted house a couple of years ago, and the makeup artist asked me to flip over so she could tease my hair. She was pulling on it so much, I just started crying. I was quiet, and my hair was covering my face, so no one knew until I flipped my hair back and my face was stained with tears. I explained that I hated having my hair pulled and I don't know why it gives me such a strong emotional reaction. She wisely told me I could do my own hair from then on.
I get very stiff and panicky when people touch my hair without asking. God forbid someone actually pulls on it, I instantly switch from normal/happy to starving pitbull angry. It's so bad I usually warn people when I know I"m going to be around then a lot and I'm meeting them for the first time (mostly coworkers)
I don't really know where it comes from. I have very vivid memories of my mom brushing my hair to get it into a ponytail and how much it hurt, but I had to just sit there and take it.
My boyfriend LOVES my hair (I really wouldn't date someone who didn't), and he knows to somehow let me know he's going to touch it, but he's very respectful of when I don't want it touched.

Of course, all of this doesn't stop me from playing with my OWN hair, especially now that I've gone CG and it's so soft