Because the book is written from Phillip's personal point of view, we are privy to his innermost thoughts and feelings. This intimacy lets us see just what Phillip is thinking about whatever situation he finds himself in.
Phillip's narrative tone changes over the course of the novel. When we first meet him, he's petulant – fighting with his mother and thinking poorly of Timothy. His tone changes, though, once he matures a bit, and we see him address those around him with respect and courtesy.