There are lots of different reasons to get out of the house and get active.
I, for one, love a good thrill on occasion and a good bit of novelty on a regular basis. It just keeps things interesting.
Then, of course, are the obvious health factors. I like to feel better, look better, and have a sense that I'm doing OK for myself.
Competition can be a factor at times, particularly for people with particularly competitive mindsets. Who doesn't want to earn a little esteem from peers?
My brother-in-law wants to get in better shape so he can be around to watch my niece grow up. I'm stuck staring at computer screens, circuit boards, and (worst of all) dashboards for far too much of my working life, and work takes up most of my waking hours.
You don't have to think about your motivations to get out and have a good time. Or maybe in your case it'll help. Either way, get up off your butt and go! ;)