I've been thinking about this for 2 years now, and I'm still not sure. Many of the reasons that can be seen as the wake-up call can also be seen as reasons not to stop. "My partner left me, so I needed to quit" can also be "My partner left, I have nothing to live for now, I might as well drink".
I wish I knew how and why I turned the internal knob. I only know that I was 100% convinced and committed. Maybe it was just survival instinct kicking in. I honestly don't know, but I'm OK with that. I'll keep looking for the answer though, as it seems so important - especially when you want to help others to kick the drink.