How boring, flat and uncomfortable would this scene be if CC simply said to her best friend Hillary: “I love you, but I always thought I hated you, so I never showed it. You don’t know how much I really care!”? It’s much easier to speak about a dog than to speak directly to a friend about FEELINGS. 

This is what we might call thematic subtext. It explores the themes of the movie: Friendship and loss while also foreshadowing what CC is going to go through later in the story, losing Hillary. One might argue that the whole exercise of watching a film is to LOOK for a theme. If a writer expresses the theme outright, they rob their readers of searching for it.