I started formal exercising when in college, and at that time my primary reason was, I think, the being-attractive-to-opposite-sex element. Also, linked to that I suppose, my liking how I look when I'm fit and slender. And, it was also a bit of a social thing, at the time... something most of the students did, and so I'd go to the gym with friends, usually.
Since then, it's been a continuation of the habit. Also, after college, I a) had a job at a national park briefly, and was impressed with a couple in their 60's/70's who were leading hikes, and very fit for their age, and I wanted to try to do everything in my power to be like them if I reached their age; and b)I watched as my parents grew older and became much less fit, less mobile, and I wanted to try to avoid that.
And, over the years, I've realized that I have come to really really enjoy being active, and I 'need' it in a sense (to the point of my dreading the possibility of something happening to my body such that I'm no longer able to do it - being stuck in a chair or bed would be really really depressing for me). Many of my hobbies are active (though, aren't team sports, they're solitary). It also really impacts my emotional state in a positive way. So, it's pretty important at this point on a number of levels.
I guess it started out as a few reasons, and has branched into a lot.