I believe the odds are vastly in favor of not hitting my head. Vastly. Many are swayed by reading/hearing accounts of head injuries or perhaps having 1st or 2nd hand experience with someone that has. By nature our brain pays more attention to danger and weights it more importantly than most other things. Staying alive has always been pretty important.
Because of that bias and the fact that the millions of people that don't hit their heads riding a bicycle, skiing or get in a car accident are not reported, "we" just see and over estimate the danger of many activities.
Media love to use that to get us to pay attention to them. Just go look at The Weather Channels website, it's all about DANGER!!!! Killer storm, see friends at risk!!! We're drawn to danger whether we're involved with it or not because it's important to our brains and our brain wants us to avoid it. Just look at the coverage a celebrity gets when they have an accident/mishap. How can people not think skiing is dangerous when the few celebrity that have been killed or injured are so burned into our consciousness?
Laws, social pressure, the desire to 'fit in' [another driving subconscious force] and our memories [which are pretty asymmetrical in the information they take in at best] control most peoples "decisions" on the merits of a safety device.
Personally I would never tell someone over 18 to wear a seatbelt or helmet nor would I think them foolish to do so or not.