What reason does Brinker give for Gene's decision not to enlist?
Brinker claims that Gene feels pity for Finny, because he caused Finny accident. Before Finny returns, Gene is planning on enlisting with Brinker, & they agree to enlist together upon graduation. Because Finny is gone, Gene is more open to the influence of others, and Brinker's rather heavy-handed accusation of his guilt in Finny's accident makes him ready to go along with whatever Brinker suggests. Without Finny, Gene feels as though he has no reason to remain at school, & enlisting begins to look better & better.
However, once he returns to his room, & finds Finny there, he's suddenly found his reason again. He feels he must stay at school & take care of Finny, helping him through life. Finny, of course, finds his meaning in training Gene for the Olympics. Yet Brinker ruins this happy reunion by reminding Gene of his "little plot", & suggesting that he is remaining behind only out of pity for the way he's changed Finny's life.