Maturity
The only true cliche about the military is you do grow up in a hurry. Even without combat, or even without overseas duty, the very act of being a meaningless cog in an organization that is barely aware you exist can give you a whole new perspective on reality. Not school or even an industrial job is so capable of quickly giving you a sense of total alienation. You suddenly realize you are not the center of the universe.
Its not that difficult, most people get through the experience OK. But no one gets through it unchanged.