From the perspective of 49, I think that part of it is just watching the body count pile up over the years. When you're 18 if you're fortunate you haven't lost many friends to any causes at all, but as the years go by you see them go by car wrecks, plane wrecks, motorcycle wrecks, drug wrecks, self-inflicted wrecks, wars and any number of diseases. It gradually dawns on you that it COULD happen to you. Still later it dawns on you that it eventually WILL happen to you, but there's no point in stacking the odds in the reaper's favor.
The wonderful thing is that, as the adrenaline urge subsides, life offers so many other pleasures to take its place (and hopefully more money to afford them). Good scotch, fine wine, good food, sunsets, pretty women (taken in moderation) etc., etc.
Did I mention good scotch?