I'm not sure anyone knows, but I've seen it on plaques, posters, scribbled on walls, etc, from the time I entered the Army in 1970. I think it was one of those generic sort of sayings which came out of the RVN War, and the closest quote I've ever found with an attribution is this one by John Stuart Mill:
“Life has a certain flavor for those who have fought and risked all that the sheltered and protected can never experience.”
Richard