social capacity

It means that the motocross rider started doing it for fun, with a group of friends. (He enjoyed it for the social aspect, which means spending time with friends, talking, laughing, sharing experiences.) Then he got very good at it, evidently.

I just realized you meant "career" when you typed "carrier." The interviewer (the person you're quoting) is basically saying:

"You started riding just for fun. Did you think that it would turn into your job?"

Career = profession, job, a way to support oneself by earning money.

ahh, sorry about the carrier thing:)

and thanks for helping me!