It took millions of men who had never been far from home at all and sent them thousands of miles away to places they had never seen. It aged them, quickly and brutally, by exposing them to carnage, destruction and death on an scale difficult to imagine.
It made them appreciate life, and led many of them to try and live it to the fullest when they returned home.
But it was difficult for some to live with the things they had seen and experienced, much less explain them to loved ones in a way that they could understand. So many veterans of that war, my grandfather included, just buried it, and never spoke of the conflict until much, much later in their lives, if at all.
PTSD was undiagnosed at the time, and society did not adjust well to those unable to mentally deal with civilian life in a post-catastrophe world.