From Mark Helprin’s A Soldier of the Great War:
He knew that this was because the war was still in him, and that it would be in him for a long time to come, for soldiers who have been bloodied are soldiers for ever. They never fit in. Even when they finally settle down, the settling is tenuous, for when they close their eyes, they see their comrades who have fallen. That they cannot forget, that they do not forget, that they never allow themselves to heal completely, is their way of expressing their love for friends who have perished. And they will not change, because they have become what they have become to keep the fallen alive.
This is as good an explanation of what it means to be a soldier as I’ve ever read. It rings so true to me, especially when I close my eyes at night.