All the responses are correct. These days it's a complicated web of various factors, but the academic term I was taught for it was social caging (yeah, I know I probably misspelled that). In short, the state seeks its continued existence (and its existence is solely dependent on the violence of the military and police (and yeah, I know, I'm oversimplifying it)). Therefore, it uses various means of social pressure to encourage military service. Honor, glory, duty, citizenship, etc. The book Starship Troopers puts this into perspective quite well, very much worth a read (completely different from the movie from what I understand) (and no, I don't agree with the society it puts forth, but I really don't think the author does either. Read The Forever War afterward, a book written almost in response to Starship Troopers, they are strangely two books best read together).