Why did James Castle jump out the window at Elkton Hills?

Phoebe tells Holden that he doesn't like anything that happens. She asks Holden for one thing that he likes a lot. He thinks of two things. The first is the nuns at Grand Central. The second is a boy at Elkton Hills named James Castle, who had a fight with a conceited guy named Phil Stabile. He threatened James, who responded by jumping out the window, killing himself.