Right in the middle of my department refresher for SCBA, I turned my mask in. I had completed my refresher, scoring well and passing with ease. Something just "popped" in my mind and said I was done. I turned in my mask and interior tags right then, and never looked back, I did stay on as a exterior firefighter.
Same thing happened with my state EMT rating. I took the whole refresher course (was far from the first time), and when it came time for the practical (I should have very easily passed) I just said I was done. I don't know why, but I just was. I've missed being a EMT a number of times, all in all do not regret it.