I make Pokémon hacks.. well, I ROM hack because it's an easy way to let your creativity flow, as well as being an avid fan of the game that I'm hacking. Whenever I'm playing a game, I always think: "What if ___ was there?", "What if ___ looked like this?". My mind is always thinking of new ways gameplay can be projected to the player.
Also, I've always wanted to create my own video game. And 'hacking' a game basically makes it as 'official' as possible; without having to hire coders/programmers, everything is given to you to make your own game based on what's there. The possibilities are endless, it's a pass-time, and it's a hell of a lot of fun :o)