Two things did it for me, at different times.
In 2004, I was head over heels (or so I thought) with a girl who didn't feel the same way. I was probably about 190 at the time.
Some things happened, and basically that summer I used the hurt/rejection as motivation. Began hardcore diet experimenting (low carb, psmf, even the All Natural Peanut Butter Diet - yes, there is one, and it does work!
) as well as hitting the free school gym 3x a week.
By the end of the summer I was 172, and with the weightlifting I was looking pretty good. Then school and work started in full force, and, coupled with me getting a steady girlfriend, I slowly let the weight creep back.
Flash forward to Christmas 2005. I weighed 210 lbs, but oddly enough didn't really show it. But my aunt (who can often open her mouth before thinking) made an off-hand comment, something along the lines of, "I guess you've been hitting the sweets again, eh?"
I felt awful, angry and disappointed with myself for pissing away all my hard work.
So, at 210, in February of this year, I decided enough is enough. I stared a PSMF and got down to 190. Two weeks at maintenance made me hit 195, but that was water weight.
Then I started low carbing, coupled with 3x weekly runs. As of yesterday, my morning weight is 168lbs. My goal is 160lbs, but I'm looking into a gym membership to start a true body re-composition.