But I know a base jumper .. and he only does the jumps if he feels the fear and his kick is to overcome it and feel the adrenalin rush.
This sentence has beautifully crystallised the meaning of what it means to be an adrenalin junkie ^_^
I can identify with this “adrenaline junkie” definition, with qualifications. It’s probably different for everyone, but for me it’s more about “playing the edge”. I’ve crashed badly in the past going over that edge, but my-oh-my is it beautiful to approach, and get as close as you can, and look into the abyss for a moment. When the fear becomes too much, you back off. Over time (and survivorship bias!) you grow to have an immense respect for that edge.
Now, when I (rarely) go past the edge, I’m not flying past it to my death, my approach was appropriately calculated to produce a recoverable step-down or at worst a minor side-fall. I haven’t gone over the handlebars in years.
EDIT: People are a complex blends of emotions and motivations, so you're certainly right that can be another explanation for the same observable behavior. I really liked the comment about adrenaline junkies too. My point was only that on average it's low-sensitivity people who engage in those sorts of activities. Scaredy cats like myself stay home and read a good book.