I started riding bikes after a serious motorcycle crash that prompted me to give it up. I've been tempted to try some mountain biking but I've held off mainly because the people I know that do, are often not riding at all because they're recovering from some kind of crash; broken collarbones, bruised or broken ribs, concussions, etc. As we get older, recovering from broken bones, etc. is just a lot more difficult and I find I have less tolerance for risk than i used to.
At the moment, I get all the enjoyment I need road biking so I'm content to stick with that. Naturally it has it's own risks, and I do my best to minimize them.