A lot of things, many mentioned by pps.
Sometimes the people who choose to join the forces don’t have lot of options. So whether that is having been int he care system, coming from a deprived community, coming from a dysfunctional family.
Then the forces are very institutionalising. A very regimented life with lots of routine and order. Which sometimes doesn’t equip people int he best way for life after service. Coming out of the forces into everyday life can be bewildering. Things that were taken care of automatically before suddenly become the person;s individual responsibility.
And the leaving the forces also sometimes means leaving a lot of friendships and support networks behind. People don’t always have those networks in civilian life- maybe the life they had before the forces never really contained those, maybe moving round a lot during service meant never really having the time/opportunity to build those up outside the force.
And that’s without things like PTSD, or physical injury or other health issues (physical or mental) caused during or exacerbated by service.
I also think that civilian life just is very different to frocesmlife. I shared a flat for a while with a pilot, who had previously been in the RAF for about twenty years. He was relatively unscathed, supportive had done a degree before serving, so he was well equipped in a lot of ways for everyday life (not least high earnings).
But his marriage broke up soon after he left the RAF. They’d gotten used to him being away a lot (she had stayed in UK to give kids a stable life) and they couldn’t adjust to living together all the time when he got back.
They had enough money to run two households, but I think a,lot of other forces couples wouldn’t. And relationship breakdown is a major cause of homelessness.
One night, after a rather wild night out I went on with him and his airline colleagues, he broke down in tears when we got back home.
He told me how much he missed his fellow flyboys. That he felt like they always had his back, that he could really trust them. That knowing that they were all prepared to die to protect one another had made him feel safe in a way he hadn’t felt before or since. That he belonged there, and that civilian life was so different- people squabbling and backbiting, never really knowing who you could trust.
In the air, it was clear cut for him who the good guys and the bad guys were. But everyday life was just a mess.
I don’t know if what he experienced was trauma bonding, or just simple, deep, straightforward camaraderie- you depend on others for your life as they depend on you for theirs, so there is no room or time for nonsense.
But he definitely missed it desperately. He looked for something to replace it in a lot of places- some good (loving relationships), some bad (alcohol and sex addictions). But I don’t think he ever found it. Closest he came when was when he had another child, but his relationship with the child’s mother had already broken down.