For me, it was more gradual. Still, if I had to pick one moment that told me "this is it!" it occured after I decided I needed new work clothes. After losing my job, I couldn't stand the idea of going back to a job which would (once again) require any type of a uniform. Which meant I needed to trade in my standard jeans and t-shirt fare for something more professional. Unfortunately, since the time when I had last worked a non-uniform job I had grown at least a size. Which meant none of my clothes fit.
I was a size 20. And, unfortunately the "normal" stores in the mall don't usually carry clothes going past an 18. If you're lucky. So, I went searching on the internet. I came across two online stores which sold clothes in my size. I promptly bought them and when they arrived I discovered... they still didn't fit. That's right, I no longer fit a size 20!
It was then that I realised something HAD to change. It was bad enough to be beyond an 18, but to be OVER a size 20 was unacceptable. That meant I simply could NO LONGER shop in ANY of the normal stores. Only fat-lady stores for me. NO THANKS!
Thus, I began my quest for fitness. To lose weight and gain back the health that I'd long ago left behind.