This is something that was printed in a 1986 issue of The Front Line, a newsletter for World War II reenactors. It preceded an interview with a WWII German Army veteran, and is the most convincing explanation of it's type I've seen. (I don't necessarily agree with it, I just find it convincing.) It was written by Dr. Edward Franzosa, the editor of the newsletter.
If his country is at war, a young man joins the military for three basic reasons:
Duty and Patriotism -- Everyone with their head screwed on right feels a love for their own country and a duty towards protecting their homeland. It is much like your family -- protect it first and ask questions later.
Comrades -- When your friends are all going off to fight the enemies of your country, you want to do the same. In many cases a whole group of friends would enlist together on the basis that they can stay together and fight together. The US Army in WWII made special provisions for this. Many men grow up in families where military service is an expected duty of each young man; I know, for my family was of this kind.
Adventure -- War is the most exciting buisness there is. Nothing, nothing at all, can compare to the feelings that one experiences in combat. (If you haven't been there, you just don't know what I'm talking about. I have, and I, like other veterans, will never forget.)
A young man in Germany feels the same emotions that a young man in the US or England or Russia felt. His motives for fighting for his country are just as right, just as pure, and just as proper. If you were not there at the time, then you do not have a good basis for questioning his decision now. Monday morning quarterbacking 45 years later smacks of hypocrisy.
When I was in Viet Nam we had a drinking toast from an old Navy legend: "My Country. May she always be right; but right or wrong -- My Country!" It was valid in 1969, it's valid today, and it was valid in the 1940s, too.