I think the military thing has two main reasons.
1. The standards for joining were relaxed post-9/11 because the US needed more people. There's a lot of things that wouldn't of flown back in the day that are allowed now because the need for bodies rose so high they were letting in people who had no business in the field or even really at a military desk. You have more kooks and quacks hanging around now who have weird power fantasies about being a solider that lends itself to more fantasies down the line when that doesn't work they way they wanted. "I wasn't a solider after all! I was a woman all this time!"
2. In a similar vein, people sometimes join for the "wrong reason." They failed as men in their view or their parents' view or society at large's view prior to joining and think somehow that the military will make them a stereotypical man. This of course will fail inevitably because most people don't even have a clear definition of what it means to be either gender. This leads them to believe in their failure to be the man they envisioned the military making them, they must of been a secret woman all along.
Ex military here:
There are tons of people who sign on who shouldn't (me included). The problem is that recruiters' have quotas, so they will try to convince as many dumb kids to join as possible, and as long as that person graduates boot camp, they don't get in any trouble for getting the wrong people to join. There are loads of misfits who join because they feel that they simply don't belong anywhere else. I spent four years stressed and depressed, and it was a fucking miracle I made it through the end of my contract without getting into trouble or attempting suicide. Thank fuck I never saw combat! I knew tons of people who resorted to drugs and suicide attempts (thankfully, no one I knew was successful with that) just because they couldn't handle it and signed up because "free college and you can see the world".
Guy I worked with for the last few months of my enlistment was a former recruiter and he even said that statistics show that if someone joins because the military contacted them first, they are much more likely to leave when their contract is up, sometimes even sooner, compared to someone who walked into that office simply because they wanted to join. Recruiters are basically used car salesmen.
Why did I join? Recruiters from every branch got a hold of my contact info and kept calling me until I finally caved and met with one. They lied to me and said that it was just another 9-5 job and that I would have plenty of free time to pursue my own dreams of becoming a musician. "You're depressed and hate your job, so why not join?" They set me up with a job I hated even more than the one I had, because it wasn't their job to set me up with a career that was actually right for me. I was just another dumb kid who thought he was going to be a badass.
The military simply does not care who joins. As long as you are able-bodied, aren't connected to any terrorist groups, don't do drugs, and can pass the ASVAB (which, holy fuck, is that test a joke! There are some dumb motherfuckers in the military), they will be more than happy to send you to war.
TL;DR: US military has no standards for potential recruits, made worse by recruiters who don't give a fuck and are just trying to meet quotas.