First time I quit, I had been planning it for a while but no real action. The day followed by ~2 years of sobriety was when I made a post here on SR, still drinking... and the responses I got gave me an extra boost of motivation. It was quite spontaneous, wasn't even after the worst drinking episode ever.
After a relapse early this year, I got into an ugly binge pattern. I went to rehab and stayed sober for about a month, but then drank again. That scared the *** out of me and I have been sober since.