The thing is though that HM Forces give structure, self discipline, help you gain qualifications, have a clear career path and help develop your skills.
The other thing, though, is that unlike every other job, where your personal health and safety are protected by law, there is no guarantee of this in the military - indeed, in times of war, your personal health and safety are almost guaranteed NOT to be protected. Of course, your life can be put on the line for any cause, even one with which you may vehemently disagree.
If you have given it a great deal of thought and actively chosen to go into the military for your career, then all well and good; but it should never be something that you drift into by default - especially if you're effectively told that, although nobody else thinks you're any good, you're good enough for us (or indeed, as PP have said, you might turn out to be rejected by them too and then where does that leave your sense of self worth?)
I'm not speaking for anybody else, and if he really wanted to do it, having considered it carefully, then I wouldn't stop him, but I would be utterly devastated if my DS wanted to join the military. I don't mind what (moral) job he eventually chooses - teacher, bin man, accountant, supermarket shelf stacker, nurse, shopkeeper, web developer, mechanic - whatever (and I'm not here saying that joining the army is immoral), but I personally would be distraught if he wanted to sign up for the forces. Not judging anybody else's choices or wishes for their own DC, but that's my honest, genuine feeling.