why do the boys invent stories of leper as a war hero?

Led by Brinker, they mockingly envision Leper as a war hero. Gene himself notes that by talking and laughing about Leper's heroics, he and his classmates are able to personalize the war.

"In a rather remarkable way, Leper becomes more important and is taken more seriously by the boys once he has left and gone to war. He is a symbol of the heroism and interesting deeds of war, a representation of all the brave and wonderful successes of the American troops. Their support of "Leper" is another way to show patriotism and feel involved in the war, without having to face the harsh realities involved in the real events."