I kinda fell into all of this. Long story short, I lost 22 pounds by accident and decided that if I could do *that* unconsciously, I could do a helluva lot more if I gave it a little effort. A little bit of calorie counting has snowballed into a life changing obsession :)

Because it challenges me. I actually have to try, and there's no end to the things I can do/improve/change. It makes me feel like I'm living my life consciously, rather than just floating through it.