Fear and the lack of it
Repeated exposure to fear finishes the fear itself.
Years ago, I used to do adventure sports because the excitement of being able to overcome my fear was exhilarating. Now, I do them just 'cause they are fun. Last weekend's parasailing was calming and serene. Waving to the kids down below was like sitting in an airplane and waving. I didn't experience even an ounce of fear.
I guess this is what drives old people to keep looking for the next exciting thing to do. To generate that fear is now the goal not the abolition of it.