Why soldiers fight is as individual as the soldiers themselves. Some don't even know why they are where they are, or what they are fighting for, except the government told them they had to be there. There is a difference between the ideals of a Volunteer army of 1861, and those of a Conscription army of 1862 and beyond. The same with our Army in Iraq and the Conscript army of 1968. That is not to say that there are not patriotic soldiers in a conscript army. But most soldiers in a conscript army are there because thay have to be, and not because they want to be. Most conscript soldiers are not fighting for any idealogical reasons, one way or the other, they are just fight to stay alive as long as possible.
But you know this Sir, since you have been there and have seen the elephant. You know why you fought, and I know why I fought. But, I can not say why you fought except by what you testimony say you fought for. The same is true of "The Gray Book". I do not know why any of them fought individually except by what they have written and the testimony that they have left behind.