Two things propelled me to pull the FIRE trigger:
1. I was working on a very detailed doc for a legal client of mine, only to have the f*rm's computer system crash while I was putting on the final touches. The doc had taken about a week to compile. Even though we had an allegedly sophisticated computer system, the doc could not be retrieved. When I was told that, I had a searing pain in my skull. I said in a very loud voice (my off*ce was near the f*rm's lobby) "I DON'T NEED THIS. I CAN LEAVE WHEN I WANT. I DON'T NEED THIS..." Everyone who heard me was flipped out. (Turns out that a much-later xray showed a now-healed aneurysm in my brain. That pain in my skull that day was likely the cause of it.) In any event, I thought at the time: This is not worth it.
2. I was then celebrating my 50th birthday on a vacation with my SO. We were walking in a generally beautiful resort, when, for no particular reason, an epiphany hit me near an ugly old parking lot. I just stopped, turned to my SO, and said: "I'm 50. I've worked hard for 25 years! I DON'T OWE ANYONE ANYTHING. I AM GOING TO QUIT MY J*B WHEN I SEE FIT!" And my SO said "Fine!" And off we walked...
I then set the date, 18 months later, on which to depart the f*rm. That gave me the chance to ensure that when I jumped, the net would likely appear. I replaced my roof, HVAC, etc. on my paid-off house. I bought my first bond positions and set aside a wad of cash by selling some stock. Etc. Etc.
After I ER'ed, we went back to that resort again, and had an even more posh stay -- the nicest hotel, great restaurants... AND I took a photo of that "Epiphany parking lot" which I have to this day!
So there were abrupt moments, and a little careful planning too.
And I have no regrets. At all.