Because of The Watch House, by some way the longest and for emotional investment best game I ever GMed. I fully expected it to run for about twelve weeks, not six years.
A simple premise, lifted (with permission) from SteveD. Where do Watchers come from in Buffy? Their university years are sure to be pretty formative. Steve set his game in Oxford, so I put mine in Cambridge. Since the show had just ended, I set it five years in the past rather than dealing the fallout of season seven. I joked that if I was still running it in five years I'd have to worry about that... and sure enough, I was, and I did.
A lot of its success was lightning in a bottle - the mix of enthusiastic and creative players throwing in ideas - but there are some lessons a GM can take away.
The Buffyverse is like any superhero universe, open to a wide variety of stories, starting with action-horror-coming-of-age-in-a-funny-way but spreading far and wide. I ran almost-serious ghost stories, love stories, family dramas, flashbacks, flashforwards, perfect worlds, nightmare futures, broad farces, fairytales, killer robot B-movies, multiple-versions-of-a-character adventures, dungeon crawls, descents into the underworld both literal and figurative, and even a behind-the-scenes episode.
The NPC cast may have gotten too big at times, but a fair number of the NPCs were well enough defined that different PCs had different views on them and each had a point.
A system I could run in my sleep (and very nearly did in one convention game when I had a cold) helped too.
Since then I took a year and a half off from big all-in kitchen-sink games, purposely limiting settings to certain styles, and that was interesting... but also generated lots of ideas that wouldn't fit.
Since January I got back on the do-anything bandwagon, and it feels rather like coming home.