Why does Chris's relationship with his family deteriorate?

He spends the summer after his sophomore year delivering pizzas for Dominos, keeping careful track of his earnings, and becoming more withdrawn from and hostile towards his parents, for no reason that they can deduce. It turns out that the smoldering anger is fueled by a discovery Chris had made on his road trip after graduation, when he had gone to the neighborhood where he had spent his earliest years. Here, he gets enough information to piece together that his father had continued his relationship with his first wife in secret long after falling in love with Billie and fathering Chris, even fathering another son with her. As Chris’s resentment towards his parents grows, so too does it towards the rest of the world, as he starts to, for example, complain constantly about all the rich kids at Emory.