For me, I don't think I had a true "a-ha" moment. As my weight creeped up, I kept developing more health issues and had to accommodate my growing size for nearly everything in my life. It's funny how we just seem to sort of accept these things as a new reality.
Then, one day my husband and I had tickets to a concert at a local arena. We were walking from the parking area to the entrance, which was a block or so away and required us to cross a couple of intersections. The group I was with walked passed a man standing near the corner under some trees. The man yelled out some remark about people needing to lose weight. Whether the remark was meant for me or not, I don't know for sure. BUT, it FELT like he was talking about me. His remarks made me feel ashamed and really bad about myself. This was an entirely new experience for me. I had never had a stranger say ANYTHING to me about my size.
I can't say that I went into action right away or anything, but it sure got me thinking about how far I had let myself go and that I had to do something about it. A few months later, I started seriously contemplating surgery. So, I'd say that jerk on the corner of the street was my catalyst. I still think about him today, but not in the same way as before where he made me feel bad. I sort of look back on that memory and use it as a kind of motivational factor.