Cameron knows, objectively, that he is skinny. The mirror tells a different story. Tommy knows, objectively, that the world is survivable. His head tells a different story. So when they find themselves in Brooklyn High School's Most Screwed Up Kids group (not official title), with little in common but a shared tendency to self destruction, it can't do any harm to strike up a conversation, right? And if it'll get John off Cameron's back, maybe having a friend would be good for him.