I'm a cynic on this subject, but my first thought when I see a young or youngish person do something like joining the Peace Corps is that they're resume-padding. A lot of that is because pretty much everyone I know who's done some kind of long-term charity work clearly was doing that (went off to the third world because they didn't get the internship/job they were applying for, or are angling to get into DC/politics or somesuch).
As long as I'm making controversial statements, you can probably extend that to people who join the military in a role that isn't likely to see combat and who don't intend to make a career of it. That's a lot less common than the Peace Corps, Teach for America etc. amongst my very non-representative social milieu (urban, East Coast, liberal), however.
