This reply was beautiful, thank you.The decision to stay or go is very personal and often flavored by your experiences during those 5 years. The rear view mirror of regret is then flavored by how life went AFTER you made the choice to depart and is the classic dilemma of the road not taken and FOMO. Did the assumptions you made about the alternate choice pan out?
I would say in my own case, I made faulty assumptions about the private sector that did not play out as anticipated. It could be the game -- or more likely it could be the player. Much of my experience is related to the small-business direction I selected coming out of the military. I recognized the error of my ways quite early, but could not return because of larger force reduction initiatives at the time. In all fairness, there were people in my life tried to point out the benefits of remaining in the AF. In retrospect, I was not listening because the grass appeared to be far far greener in the civilian sector.
In the end, we make big kid choices that have rewards as well as consequences. We get one chance at this game, and the rear windshield is far more clear than the front windshield on the rocketship we fly during our limited time here.
For some, they were never a fit for the discipline and process orientation required from a military lifestyle and I have to think there is no remorse for separating after 5 years of service for those in that category.
For others who enjoy the camaraderie, common goals, common struggle, order and discipline, clear growth plan, achievement mindset, etc... it is often something that cannot be found in other professions. Often, as a ~27 year old human, you don't fully know yourself as well as you do as a ~40 year old human. Perspective shapes what you value and if you are able to learn this about yourself sooner, you will often make better longer ranging decisions that align with your true values.
Everything I wrote above only answers part of your question -- and again, the answers vary SO WIDELY based on each individual's circumstances and background. As for reasons to stay or go, one reason to stay would be resume builder. Military officers are asked to do far more than civilian counterparts at a much earlier age. The longer one is in the military system, the more responsibility they are asked to take on. There are some examples within structured corporate managerial training programs within the civilian sector, but it is not the norm.
An example that hit me the other day was seeing a job posting ($400K/year role) that asked two key filtering questions:
1) What is/was the largest team you have led or managed?
2) What is/was the largest budget you have been responsible for?
Within the civilian sector, large teams and large budget are possible, but not as plentiful as a mid-to-senior military officer. If you consider a 20 year military officer who is an O5 or O6, they would likely run a 200+ person organization if not into the thousands. Their budget would be in the 10's to 100's of millions of dollars. With a high degree of certainty, the person who stayed in the military longer would have better resume bullets in these areas than the grad who got out as a Junior Military Officer and went the typical civilian manager path. Yes, there are other criteria for a role like this, but I point out these two criteria as one example of why someone may opt to stay in the military to build a resume in anticipation of a second career outside of the military.