1) Working at a fast-paced startup as an early employee. When I was in my 20s this was awesome. But as I'm in my mid-30s now, I couldn't really hack the 10+ hour days with the expectation that I also work weekends. There was all the "talk" about work/life balance from leadership, but that was total bullshit.
2) My wife was pregnant (and now we have a kiddo), and I recognized that I did not want to be working this hard as a new dad - I quit with no plan several months before our child's due date. I'm sure I would have been fired soon-ish with how my attitude had been souring.
In less than a few weeks, I had people asking if I wanted some contract/consulting work. I said yes, and have since slowly grown my billables to about 25 hours of work a week to have a sustainable salary.