I have left 121 several times. I didn't leave the career, unless by "leaving the career," you restrict it to 121. A career in aviation covers more than 121 flying, and there are a lot of other things to do in aviation than fly 121. I have made the change for various reasons, but in my opinion, the change was for the better.
I'm sure everyone has their own reasons. Life changes and circumstance often dictate what's best for any one of us, and that "best" may evolve with time and need. What was "best" yesterday is not necessarily so, today.