I believe some people are attracted to dangerous sports for many reasons, but  I believe some of these reasons are; a genetic disposition to take a risk, a social stimulation when they share their experience, and a detachment from life. 
A high-risk athlete must enjoy adventure. I believe that their physiological conditions make them feel something really strong and special when they are in a situation without control. They hormonal system must help them to tolerate high doses of stress helping to transform those feelings into something exciting instead. 
Another motivation that high-risk athletes have, is social approval.  They feel good sharing with others their adventures and how they have saved their lives in a critical situation. They may have a stimulation when people admire them, or maybe his self-esteem increases when they are seeing for others as heroes. They must like to feel different from others and feel they are unique or special. They want to present themselves as brave and passionate. 
Finally, I believe that those athletes don't have a real attachment to life. It is possible that this kind of people feel more alive when they are in danger and they are careless. This is something really positive because they enjoy life in a full manner, without so much restriction and regulation as regular people.