The mood and atmosphere of John Green's The Fault in Our Stars is set early in the opening of the novel. Readers immediately come to know the protagonist, Hazel Grace Lancaster, through a very direct characterization: she rarely leaves the house, reads the same book again and again, rarely eats, and spends all of her time thinking about death. Her mother, because of her activities, believes that she (Hazel) is depressed. Most readers may not come to question this since Hazel has cancer.
Eventually, Hazel meets a boy (Augustus Waters) who possesses a completely different outlook on life. His outlook on life, the "roller coaster that only goes up," proves that there is another way to look at life. Hazel, intrigued by Augustus, begins to take part in her support group (albeit to simply add humor). The remainder of the novel follows Hazel and Augustus through their battles with cancer.
Although the topic of cancer, especially in teens, is depressing, the novel illustrates that life is what one makes of it. Hazel comes to find out that Augustus loved her, and she is able to come to terms with her place in the world (defined by Augustus). The mood of the novel, while at times is rather depressing, is hopeful. The times which bring light into Hazel's life (Amsterdam and walking without her oxygen tank) show promise for her (although she cannot walk far without the tank). The mood and atmosphere change as Hazel does. When she is having a good day, the mood of the novel is upbeat. When she is having a bad day, the mood of the novel is depressing. This allows readers to really "see" what it would be like to live with cancer (figuratively).