What was the deciding factor that made you decide to leave the military, stay for the full 20, or come back into the military?

I left the Army once before about the time I hit 10 years of active service (around 14 yrs total service). For me that was the crap or get off the pot moment of my military career. Honestly at that point I was burnt out and pretty miserable. There was no way I could hang around for another 10 years. With the support of my then fiancee, now wife I decided to get out and head back to school. At that point I didn't really have any intentions of coming back in, but when I was given the chance to attend one of the Army's grad school programs as an Officer I couldn't pass on the opportunity. Some days I love it and some days I remember why I left the first time around. By the time I finish my program and the accompanying service obligation I'll be just a few years short of 20 so I imagine I'll stick around. At that point, who knows?

I decided after the birth of my son. I went into IRR and was struggling for several months trying to transition to the civilian world. Turns out all of my experience in ancient construction equipment didn't get me any higher in the unemployment lines. About 10 months in I had a good job and things were starting to turn around. Then King George the 3rd decided to involuntarily transfer me to a unit across the country that was under stop loss, so I was forced a year after my ETS. I had re-upped once so I ended with 10 years. I have to say honestly the last stint put a sour taste in my mouth.
I had passed up some promotions early because I decided that I didn't want to take them from people who knew they wanted to make this life a career. I honored my obligation but felt betrayed at the very end when I was close to being done.

I stayed for the max time. But, the thing which almost made me leave was the “normal socializing” of the military. The standards were reduced and training eased. We were beat down on classes on how we need to change our way of thinking. Forget how to stay alive in combat, wasn’t a priority any more. Learning how to not hurt feelings, being fair and embracing your emotions became out focus. Total BS.

For me, I'd have to say the deciding factor was a lack of certainty.

I didn't expect to do the full 20, but I wanted a 2nd enlistment. I achieved all my goals in the military, so now I just wanted to reenlist because I enjoyed what I did. Put in my reenlistment package. Played the waiting game. 30 minutes before I'm about to check out on a month's worth of leave (because I only took leave once a year), I get a call from the career planner's office.

Your MOS is closed, the few spots available have filled up. If you want to stay in, you'll have to lat move into a different MOS, but it'll require you to route a brand new package.

By the time I would have gotten back from leave, I'd have 3 months left until my EAS. I'd also have 3 months left until the birth of my 2nd daughter. I needed a bit more certainty in my life than a "maybe".

I was bringing a child into the world, I needed a job. So I spent my leave polishing the resume and when I got back from leave, I routed my Terminal package and took 2 whole months of Terminal.

With the amount of leave I had, I was pretty much out of the Marine Corps the moment they told me I'd have to route a brand new package to stay in for another tour.

Had I re-up'd in '71, I would've wound up right back in Vietnam. One tour was enough for me.

I stayed for 20+ because I knew from a young age that is what I wanted to do. I retired due to various health problems and burn out from back to back deployments. Sometimes I wish I could have done another 5-6 years but such was not meant to be.

December 4th, 2014. My unit from fort hood was taking a PT test that morning. It was pouring rain and the field was flooded.
The PT field selected for the test was on west fort hood, and was in the low part of a baseball field.
One NCO approached our first sergeant and asked to move the test to the higher ground so we wouldn't be flooded, the first sergeant told him to stay in his lane and that we were taking the test there, because that's where he scheduled it.
I'm not exaggerating, when we did pushups our hands were completely submerged, when we did sit ups we were taking on water down the backs of our collars.
Rain continued to pour throughout the test. Everyone was soaked and miserable on the run.
I decided then that I was going to get out of active duty when my contract expired.. now I'm on the GI bill and chilling in the guard

I processed a lot of discharges during my time in. I asked quite a few why they were getting out. The overwhelming response was, "My contract is up.". For the ones that I thought would give an honest answer, I asked, "Would you have stayed in if your contract wasn't expiring?" Probably 5 0% or more that was older than 22 told me they would have stayed in. So the answer for a lot of people, at least in the Navy, was that no one told them or convinced them to stay in. They weren't upset or dissatisfied but just figured it was the thing to do since the contract was expiring.

I stayed for 23 years.

I didn't like being a field artillery officer. It was a go nowhere field with very little applicability in the civilian world. I hated staff work. Too many officers were out for themselves and would stab their peers in the back for the sake of a positive bullet on their OERs. Finally, I worked for people who were abusive. The guys from West Point were the worst.