Oh, that's easy... There has been one thing that has been hammered into my head over the last seven years, and that is that nothing is guaranteed in life. Being youthful does not mean you're invincible. It does not shield you from possibly dying the next day, or developing life-threatening and/or disabling diseases... Things you never thought YOU would get.
It's impossible to fully grasp this until you see it happen to people around you, or even... yourself. But just about everybody will feel the effects of this in one or both ways. You must never spend your time whining over stupid temporary things, and you must never pass up the opportunity to be with those you love, and let them know how much you appreciate them. That's the best advice I have for avoiding some truly agonizing regret. Because no matter what one comforts themselves with, life doesn't work that way... Things can change instantly in the worst imaginable ways. That is life.
Oh geez, sorry. Don't be too scared... It's just me being a somewhat of a break-down moment right.