Why does Leper enlist in the military?

In Chapter Nine, Gene becomes more and more oblivious to the outside world as he spends time with Finny. The impossible happens with Leper, as he is convinced by a video of American ski troops that he must enlist and become one of them. He changes his mind on the uselessness of downhill skiing, and decides that fast skiing is fine if you are in a real hurry, which a person is when they are fighting the war. The video shows a bunch of placid, attractive young men skiing down slopes and passes it off as being part of the war effort; the image reflects nothing like the real realities of war, but nevertheless, Leper is completely hooked by it. Leper leaves a week later, before his 18th birthday, enlisted in the ski troops and perfectly happy to go.