When I was in, I had a shipmate who said there were basically three types of people who stayed in the military.

Group 1: Small percentage, but they literally loved it, loved what they did, just felt right in uniform.

Group 2: Those who wanted/needed the insurance, education benefits, etc for themselves or their loved ones/families.

Group 3: Those who had no other options, and were juuuuuuust competent enough to not be kicked out every enlistment.