Four problems.\n\nThe first is that the military includes a massive amount of psychological conditioning, and can be very demoralizing. It's  basically government-sponsored indoctrination, and (if compelled) that's a potential constitutional problem because it requires people to accept/support a government-sponsored/approved message. There's also the freedom of association problem, in addition to the free speech problem.\n\nSecond, not everyone is physically capable of meeting requirements for enlistment. That means either dual standards, which is always a bad idea, or you end up with kids having a way out by choosing to be unhealthy. Worse idea.\n\nThird, the military currently discriminates based on sexual orientation. So again, you're either going to have a dual standard, or people will be in violation of the law based on their sexual preferences. "Don't Ask Don't Tell" is/was a huge disaster when the military was volunteer. It gets much worse when service is mandatory.\n\nFinally, many people and many religions do not support the use of force to solve problems. The military mindset is might-makes-right. That's why conscientious objector status exists. Mandatory service would brand everyone who didn't serve as an outside, in many cases based on their religious beliefs. That is again government-sponsored viewpoint-based religious discrimination, and that's also against the constitution.