I forge because I can let my imagination go. I guess I also do it just to make me (and others) happy.
Have you played Tetris? You know, in the beginning you only play it because it's fun but after a while you're hooked. The addiction has kicked in. Forge is my Tetris. I.e. It's a fun challenge where there's always room for improvement.
Like the rest of you I enjoy engineering different designs, not only in forge but other aspects of my life as well.
Because when I was in Halo CE, and Halo 2, I always thought that it would be freakin awesome if I can make bases and place weapons when and where I feel like. And with Halo 3, it gives me that power. Not only that, but because when I play on Default maps I always think to myself, "Man wouldn't that be better if that weapon was there instead of there?" or " The Humans are too overpowered in this Infection map." I find problems and then I fix them. this is why I Forge, this is who I am.