I think that it is certainly true for some people that they would be happier retired early, and doing volunteer work or so, or working with random things (I think MMM does carpenting or something? On top of running a blog). I also think there are plenty of people for whom that is not true.
I like knowing that I COULD quit. Actually quitting work wouldn't make me happier though, I think. Someone is actually giving me pretty good money for doing things that I enjoy. That seems better than not getting money for doing things that I enjoy, or getting low amounts of money from it. Sure, it does come with those crappy days when I truly dont enjoy what I do, but they are certainly less common than the days when things are nice.