Recently, I attended a small gaming group, and ran my game – The Cloud Dungeon. It's a paper craft, creative game with a storyline, and I've played through it so many times, you'd think I'd be sick of it.
And it's true – I don't want to play through my game. Honestly, if I put the game away and didn't touch it for a couple years, it would still be boring to me when I brought it out again.
Yet, when I run the game for other people, it's always a wonderful time for me.
Why? I think it's because game design is unique in the world of design.
Design is about making people's lives better, easier, and more enjoyable. The unique thing about game design when compared to other forms of design is this: while you might design a tool that is as useful to you as it is useful to your user – you might code an application that saves you time and headaches – games that you create will never be as fun for you as they are for the people who play them.
So, while the motivation to create a game might be "I want to play this type of game, and it doesn't exist", by the time you've created the game, you're most likely going to be sick of it. At the very least, it's going to be old hat long before it's actually a fun game to play.
Game design is unique, because the joy in game design comes from seeing other people experience joy, not directing experiencing it yourself. Maybe game design is a more empathetic form of design.
Update: Thinking about this more, game design might be a little more like an art form: creating music or filming a movie. You're making something that you inherently can't experience in the same way your audience will – you can't "discover" your film for the first time, just as you can't "discover" your game, fully formed, for the first time (unless you have amnesia). However, you do have a similar moment in game design where you "discover" the fun, and that's a, eye-opening, rewarding moment.