Why does Finny create so many fantasies?

Just as Finny becomes more and more withdrawn, his usual jovial theories and insistence on living in his own world become more unhealthy, as Finny begins to live in a great sense of denial. Finny refuses to hear anything related to the war, when before he merely jokes such talk aside, and he stubbornly insists that Leper is still around, or that he's gone off into nature to explore. Brinker also undergoes a change, losing all his determination to march on to war because he cannot find anyone else who is willing to go. He resigns every post he holds on campus, as one of the foremost figures of the school; both he and Finny pretty much stop doing anything around campus, as the school loses more people than just Leper, although the others are still on campus.