Boredom to be honest
I'd retired too early at 55, and after a few years I was bored outta my skull, only so many books you can read, TV to watch and I don't play golf!
I tried to volunteer to teach English at the local HS, but of course ran into the Thai only thing, wasn't looking to get paid, which I think is the definition of 'work'
My wife was already piss$$ed off with her job, a women engineer in Thailand who's smarter than the guys is always a problem
So we figured it was time to go
Still enjoy our time there, now as tourists couple of times a year