Started running as a Dungeon Master since the 4th edition Red Box came out. I wanted to be a Dungeon Master so I could utilize what I've learned from the game to game design experience. I'm a graduated game designer now, so I've got a strong feel for the rules and the crunch. I still enjoy running games in order to get narrative structures right.
That doesn't mean I don't like to be a player once in a while, though.