I've always wanted to have a great body, but always convinced myself that I looked just about good enough. Eventually, I forced myself to the gym and haven't looked back.

It's addictive, and it's a productive way to spend time as opposed to in front of the tv.
I feel better in and out of clothes.
Hitting a new personal best is a real good high; I have got demotivated many times, but I've realised that if I stop thinking about how I look just about OK, and I concentrate on actually hitting new PRs, you feel good both in and out of the gym.