Left because I was doing three jobs as one person. They couldn't hire a replacement with that job description (surprised Pikachu face). My new job wasn't as exciting as I thought it would be, so I got hired back in a properly defined job and that's been working out fine.
Necessity, multiple times. I had incredible luck out of college - and by that I mean that places I worked at kept closing or getting bought out by other companies and doing major layoffs. I kept going back to my fast food job I had in college because it was better than nothing. I went back 4 separate times before I found a job that didn't close within a year.
So it wasn't exactly by choice!
Hated the new one. I am glad the old one was kind enough to take me back, even if we all knew it was temporary.