Why di Gene want to enlist after the day of volunteer work at the railroad yard?

Gene wants a sense of purpose to his life, and feels that enlisting will give this to him; he thinks hard about it over the night. Gene realizes that he doesn't owe anything to either Devon or to his parents anymore; the choice of whether or not to go to war depends on his own inclinations, and he wants to go through with it. But, then he goes back to his room, and finds Finny there; suddenly, what he resolved to do no longer matters, as he has a purpose to stay at Devon again.