Hard disagree here, as this job is for enlisted soldiers. Ego and money are out on that alone. Revenge, maybe? But you really don't know where you'll deploy. Ideology... maybe as well? The Army understandably does a pretty good job of obtaining uniformity there.

Virtually all of the motivations of folks I knew could be abstracted to "things are truly bad for these people; how can we fix it?".

I think you're right