Like some of the others have said, I weigh the risk of of being hurt or worse while riding versus the risk of poor health and listlessness from a sedentary lifestyle.
But perhaps more than that it is the thrill of riding versus boredom. I have always been a bit of a thrill seeker, an adrenaline junkie. I need the experience of speed. I need the adventure of going places and doing things that many people would consider too hard or too dangerous. And for whatever reason, this thrill seeking has often involved being on two wheels. Bicycles and motorcycles have been a major part of my life as long as I can remember.
But I am not a careless person. I put much effort into learning how to control my bikes in the situations I put myself in, riding in traffic, Riding fast down hills, riding over rough terrain, riding quickly on twisty singletrack with inches of clearance to trees along the trail. Things like that. Before I do these things, I make sure I know how to do it and I make sure my bike and other equipment are up to the task. I have made adjustments to my bikes and my riding style to prevent hurting my bad back (a condition caused in great part by sitting at a desk).
I consider the risk of living a boring life as at least equal to the the risk of being hurt while riding.