How do they do it?

I was thinking,
A few years ago, my older brother (who I shall call Bob ) and I were talking, the subject matter being something I have thought about from time to time over the years.
He said something like, "How can these guys who serve in the military fighting for their country, come home afterward and go back to their regular jobs and have some goofy boss yelling at them for some stupid reason?"
He then went on, "Take someone like Uncle Fred, who fought in Europe in WWII, was in some serious battles, he even was wounded. How can he come back to his desk job and take orders, or grief from his supervisor, when just a short while before he was crouching in fox holes carrying a weapon while defending his country?"
Interesting question isn't it? I don't know the answer. The war of my generation was Vietnam and they discontinued the draft a month or so before I was to graduate High School, so I have no clue how solders due what they do and then return to their "normal" lives as if nothing happened.
I can't even pretend that I would have had what it takes to be a guy who wakes up everyday with a gun by his side.
I was 18 when I boarded my first oil tanker and I was scared senseless doing that, and no one was shooting at me!
Anyway, I was thinking of this subject recently because my future son in law, who has spent the last 7 months carrying and using a rifle as a U.S. Marine in Afghanistan, came home safe and sound the other day.
Let me tell you, there are two very relieved families in Pennsylvania right now.
And one really really happy daughter.