Back in the day, our group rotated DM's. Each DM would mess with the campaign, and there would be no continuity to the games. I volunteered to become the regular, ongoing DM, as I had really come to enjoy running the game.
For my current campaign, I decided I wanted a friend to learn the game so I would have an excuse to buy the new 3E rulebooks. And I figured what the heck, I might as well DM to since it'd be only for one player.
But then that player invited a friend, who in turn invited some more friends.
So now I've got a lot more gaming friends and a Forgotten Realms campaign that's lasted five years.
After going through "The Adventure Begins Here!" and a fairly good early-level adventure in a small town, the DM and I co-bought an adventure we didn't realise was for about 7th level.
We played that I think twice before the DM said he didn't want to do it anymore. Because none of us had played before "The Adventure Begins Here!" (Though many of us had old books from previous generations. Odd.), the DM said he'd like to play. So I ran.
Despite numerous one-shots or small, unfinished campaigns, I'm DMing.
I started GMing when I first moved to Topeka (IIRC, that was 1996). As to why I started GMing, it was because I was one of few gamers in Topeka at that time who had an interest in taking a chance on new systems (everybody else was pretty firmly entrenched with AD&D 2e or V:TM and dared not buy a new game until they'd first played it). I ended up running a lot of unofficial demos and one-shots for folks who were interested in System X, but didn't want to buy it themselves until they'd first played it.
I started DMing for the unbridled power over the minds and souls of my friends. One night with a pint of fresh blood and a bucket of KFC I summoned Cthulhu himself to do my bidding. But it backfired and I was forced, FORCED I TELL YOU, to buy one book after another of things man was not meant to know. You know, splatbooks, adventures and settings I knew I'd never use, that kind of thing. Just say no.