An RPG is a way for me to spend a few hours writing up what amounts to a basic storyline born of my imagination, and then turning my friends lose within it for days or weeks of continuous mutual storytelling that more often than not spawns even more of the same until we all have found ourselves living out the adventures we all wish we could have if the world was a perfect place.
For years afterwards we retell the stories of our adventures both from within and without the context of the continuing campaign until the smallest, least interesting adventure has become an epic worthy of Beowulf. Or at least something so hilarious to retell that it causes epic beer-spewing.
No matter how it all turns out, though, the most rewarding part for me is to watch the fun my friends have playing in my games. Its like a gift I try to give them every time we play - I can't wait to see what they do with some puzzle or opportunity that pops up as result of both the planned and unforeseen consequences of events. If haven't done a good job, I feel like I let them down for wasting their time - so it always forces me to grow more as a storyteller, writer, and the chaos they often spawn helps keep my mind nimble.
And for some strange reason I never mind when I have to throw away some carefully crafted idea or plotline because the players came up with something that sounded like it'd be more fun - not that I let them know that, of course. I gotta maintain the illusion until the campaign or adventure is over at least.