For me there wasn't one particular moment but a combination of things. I was so tired of wearing the same old clothes over and over again (elastic pants and big, long shirts). My self esteem was shot, I didn't want to go out socially with friends especially.
I was tired all the time and didn't do any excercise.
I need to remember how miserable I felt being fat and how good it feels to have control over my food and not letting food have control over me.