I had a Honda 250 Enduro I bought when I was 16. When I was 18 I was cocking-off and wiped out real good. No helmet, shorts and T-shirt. I was skun-up awful, and the bumps and bruises on elbows, knees, head, butt, hips, etc, took me out of commission for a few weeks. I was very lucky.
I tried to get back on the horse for a while, but I never really did. I'd look down and see the pavement rushing by and remember the feeling of my body tumbling uncontrollably head-over-heels like a rag-doll for what seemed like forever, the feeling of my head smashing the ground, the feeling of the bike smashing me as we both rolled together for a while...
Apparently one bad spill was enough. I rode a few more times that year, but only half-heartedly. I realized one day the following year that the whole summer had passed and I hadn't ridden once. I kept it around for a few years thinking that someday I'd ride it again, but I never did so I let it go.
Even though I look at bikes and think how fun and economical it would be to have one, that memory has kept me from it, and I suspect it always will.
My boy just rear-ended one a couple weeks ago. Lucky no one was seriously hurt, but hes about to understand what I was talking about leading up to him getting his license when I'd harp about "high-risk" premiums.