Just imagine, your 19, and this last spring you graduated from high school.
After 3 months of fun in the sun, you're ready to go to college. You pack all of your stuff and get on a bus, and then you're off to the school of your dreams. Then you blink. You were mistaken; you aren't on your way to college, but rather to boot camp. All Americans are required to serve in the military for 2 years.
By mandating all citizens, male and female, to be in the military for 2 years, you could ruin someone's perfectly planned out life. You no longer have school, family, and friends to be with in your free time. In place of that you have loud drill sergeants, smelly bunkmates, and loud wake-up calls. When those 2 years are up and it's time to go back to real life and college, you may no longer remember what you learned in that Calculus class you took, or have the same interests as you old friends.
When you finally get to reunite with your family, you may be so used to sounding off, that you don't realize that you are yelling at those closest to you.
Then there are those delicate people. While others were building playhouses and digging for worms, they were playing with dolls and reading books. In the military, they will learn to shoot to kill, and yell or be yelled at. Just a short time ago these same people refused to dissect an innocent frog, and were to kind to squish a harmless ant. Because they were required to join the military, in any war-like condition these people may freeze up, and could compromise the chance of winning against the enemy.
For every positive reason to mandate all citizens...