How does Finny's attitude about the war change after he returns to Devon? Why?

Finny is bitter, thus his attitude about heroism and war have changed. He becomes most upset when Gene talks about enlisting, as we all know that Finny is incapable of serving;

"Finny goes off on a rant about how the war is not real, it was just designed to keep people in their place, and from having any real fun. Gene doesn't believe him at all, and asks him why Finny thinks he knows all this stuff that nobody else does; Finny then says "because I've suffered," opening another big, painful can of worms (109)."