War is incredibly destructive to the frontline soldiers in so many ways even beyond killing them -- psychological ruin, destruction of land, homes, livelihoods, infrastructure.... It's something that soldiers can be very gung-ho about -- in the abstract.
In non-desperate times, normal adults do not resolve conflicts by fighting to the death; when people do kill in anger, it's usually a matter of rash action, deeply regretted afterward.
Imagine a regular working class man in Germany, told "there's a family that lives a few hundred kilometers that way -- shall we go kill them, and take over their house & land?" What do you think he'd say?
But at a macro scale, it's somehow legitimized. And the soldiers are told whatever they need to be told to transform them from humans to game pieces -- we're under attack, or threatened, or disrespected, or we need to revenge some past wrong... Keep it abstract - here's your gun!
On the other side, micro-scale again, imagine hearing that a man and a few friends have the idea that you've harmed them somehow, and are coming to aggressively confront you. Should you... hide in the woods with a gun and shoot them on sight? Or assume it's a misunderstanding, and try to find a way to communicate and resolve it peacefully?
At human scale, it's obvious; violent death is awful, and completely final. We take it seriously.
The Christmas Truce is about some of the poor "regular folk" removing their blinders for a few days, and the tragedy of it is that the juggernaut didn't even really stumble; the soldiers that we're "weakened" by fraternization with the enemy -- e.g., doing what humans naturally do to resolve conflict -- were quickly shuffled out of the way (or in later cases, court-martialed), and the war went on.
And there is no Christmas Truce story in WWII.