I'm in my 60th year and haven't been retired long. My main motivation came about because Mrs B had a serious health scare and though she came through it safely in the end, the experience made me realise that time spent together with her was more precious than anything else. Fortunately, we had been careful with money for decades, so that made the decision - if not 'comfortable' - at least do-able.
Like others, I experienced the 'holiday' aspect for a while as well as the novelty of being able to pretty much decide on a daily basis how to occupy my time. After the first three months or so, I settled into a routine that included all the stuff I'd wanted to do more of but had always felt reluctant to do during my working years.
I think that I may always have been a bit too work-focussed. Whenever I had spare time in evenings and at weekends, I would find myself thinking/worrying that I should be using it for background reading, research, or getting ahead with work-related skills and planning. I realise now it was that kind of thinking which held me back from doing stuff that was more enjoyable and probably more healthy for me!
Since retiring, I've taken up writing (even though it was a large part of my job - nowadays I'm writing fiction, not factual copy), I'm practicing folk guitar on a daily basis, cycling, walking, eating out more than ever before. Heck, I'm even playing computer/console games. I can hold a conversation with my offspring about the latest PS4 releases for the first time ever!
Would I go back to work? Yes, because I still feel I could be useful and get enjoyment out of working if something really appealed to me. But I'd want it to be very local, part-time so that Mrs B and myself could still do lots together, and (after my last two jobs) whatever it was would have to be something that did not involve even the potential for me falling back into the trap of taking work home whether I was supposed to or not.
Possibly narrowing my options down a lot there - it might never happen as a result - but Mrs B has told me I'm not to go back to that 'work-work-work' attitude ever, and she's right - I know she is.