Due to the draft (10 million according to Wartime Draft Comparisons), you may be correct that many didn't go because they felt compelled by their own motivations. However, the soldiers I've known personally each did. My grandmother sent four sons to war, and her husband reenlisted. And they all went for a variety of personal reasons, but the common factor was preservation of the republic.
For an example of a personal reason, my youngest uncle had been captured by the Japanese on Wake, so my younger father immediately enlisted in the army airforce thinking his contribution would help end the war sooner and bring home his closest brother. But he also knew about the Jewish concentration camps, and he knew that if we didn't stop Hitler, he would enslave the whole human race, aside from a few brainwashed or greedy Germans. This kind of fear awakened both patriotism and a will to action that placed their own lives at a lower value than that of the republic.
My family arrived here in 1655, and our British-born forebear was a military man. I think that story has repeated itself throughout American history: the men who built this country bore sons who knew when to answer the call of liberty. What about me? I'm not as brave, I haven't given anything. But I'm fiercely proud of my family, and even more grateful for the troops who engaged in amphibious assaults like D-day and the island-hopping war in the Pacific.