A note about why some people serve... this particular guy was in foster care until he was 17. He had 32 foster homes between the time he was abandoned by his mother at 9 and when he ran away from the state's care at 17 to live on the streets. He soon after joined the military. With no family, no money, no decent education (imagine the disruption to a kid's life to move 4 times a year), no way to go to college... what are these kids supposed to do? This is not the first time I've heard of kids joining the military because they literally feel they have no other options and are basically living on the streets. Just take a look at the foster/adoption state site for any state- the bulk of kids are over 13 and not very adoptable by that time. The military gives them a sense of family, order, service, pride, career that are otherwise unavailable to them.
And quite a few kids whose parents don't have money for college join so they have an option to pay for college and have a career.
Basically, it isn't to say that all the people in the military are disadvantaged, but I think there is a big problem of our inequality and lack of decent homes for many children yielding a large population of 18-year-olds who have few options and little family support. They join because their back is against a wall.