Why is everyone so cruel?

In the movie, Merry Christmas, that we watched today for extra credit, (which was amazing!), gave me new insight on what was going on in this war. On Christmas Eve, the French, Scottish, and Germans ceased fire and actually talked with each other and spent time with each other and learned from each other. And then on Christmas Day they ceased fire again to have a burial ceremony for the dead that were in "no man's land." All of the soldiers that were fighting, finally realized that they're all pretty much the same, just from a different country. I would say they became close to them and befriended them. You would think that because of this they would see that they should not fight anymore, and that the war was completely unnecessary. However, they still are ready to fight. Yes, most of them do get sent to different fronts, however all of the countries still go against each other. Why didn't anyone try to say something? Why didn't anyone try to rebel and stop the war and make everyone see what they saw Christmas Eve and Christmas Day?