Lack of Agency
Lack of agency is possibly the most important theme for understanding The Fault in Our Stars. This comes from the meeting of two situations that lack agency - illness and childhood. Hazel, Augustus, Isaac, and even Anna from An Imperial Affliction struggle with their inability to make decisions for themselves, travel, and experience life in ways that a normal adolescent or even adult with an illness could, demonstrating the specific cross-sectionalism and compounding of the two traits. As Hazel explores, one of the major things that causes the feeling of lack of agency is the fact that cancer is not the antagonist of the book or of the people in the book's lives because it is only made out of the characters themselves. In comparison to situations in which the characters feel like they can act out - by smashing trophies, by egging a car, by yelling at their parents - there is nothing one can aggress against in cancer besides one's own body.
The Meaning of Life (and Death)
Because the book is about youths with a terminal illness, the meaning of life and death is very important to the characters. Throughout the book, Green allows the reader to take on many different points of view on life and death through thoughts and conversations Hazel has by herself, with her mother and father, and with Isaac, Augustus, Augustus's parents, and Peter Van Houten. Most notably contrasting are the points of view of Hazel and Augustus - Augustus believes that to have a good life one must accomplish something tangible, and that after life there is "Something with a capital S" (p.168) Hazel, on the other hand, takes the tack of doing as little harm as she can in the world, rather than focusing on making a large impact either negatively or positively, and does not seem to believe in anything after life. Other characters in the story rely more or less on realism and science versus religion and comforting "Encouragements" to help them cope with the uncertainty of life and death.
Friendship
Through Hazel, Green allows the reader to see many relationships and where they succeed and fail. A major factor in the success of a friendship seems to be empathy - that is, a care for another's emotions based in mutual understanding. The fact that Kaitlyn, Hazel's friend from high school, is so awkward around the topics of cancer and death makes Hazel uncomfortable by making her feel like she is defined by those things and cannot be either fully herself. On the other hand, Augustus, who has had cancer and generally deals with Hazel's health with as much levity as possible, makes Hazel feel understood, and the fact that he even looks past her cancer to ask her about her own interests early on makes her feel as if he both can deal with her cancer well and with her as a human being.
Love
Different characters in The Fault in Our Stars have different attitudes toward love as well. Augustus seems prepared to dive headfirst into love with Hazel, not caring that she may die soon and hurt him. Hazel, however, holds out from loving Augustus as long as possible, and even after she gives in to love she does not let him know. Aside from romantic love, there is a strong theme of familial love as well, and comparisons made between the types as Hazel's love for Augustus helps her understand her mother and father's love for her - the kind that you don't want to lose, even at the chance of getting hurt. Hazel's true love back to her parents is demonstrated by her happiness when she finds out that her mother has been studying for her MSW - rather than merely appreciating them for caring for her, she truly wants them to be happy even after she is gone.
Religion
Though Hazel, the main character, is not very religious and does not look at her cancer through a lens of religion, the story has elements of religion that make it apparent that using religion is a way many cope with cancer. Thus, the book mixes religious and secular approaches to demonstrate a realistic world of youths with cancer. The Support Group is held in a church and utilizes prayer at the end of the meeting, but Hazel treats this as circumstance and fairly meaningless ritual. Furthermore, the "Encouragements" put up by Gus's mother and father are often religious in content and tone, which allows them to cope with the approaching loss of their son.
Literature and writing
Hazel and Augustus's relationship is formed through sharing of books and poetry, especially from Hazel to Augustus. Hazel has a pronounced love of books, perhaps seeing them as a way to travel and experience things she is limited from actually experiencing by her condition, and has an impressive mental bookshelf full of inspiring writers. Her sharing of these writers and their thoughts with Augustus, who previously read mostly a gory, action-filled series based on a video game, parallels her desire to share with Augustus a more comforting and realistic way of living life than constantly seeking glory. Secondly, the theme of literature and writing, especially in the plot around meeting Peter Van Houten, lends an element of meta-fiction to the book. Green starts the novel with an Author's Note that reminds the reader that the book is fiction and not based on any person or event in specific, obviously referencing the life and death of Esther Earl to whom the book is dedicated. Green furthers this point of the separation of fiction and reality through the character Peter Van Houten, showing that he did base some of his book on his own daughter, but that his persona is quite different from his narrative voice and having him a harsh sermon to Hazel (and to the reader) on how one cannot look for answers besides what the author has given.
Technology
Peter Van Houten, an important character to the book but one whose views are often (rightly) questioned, refuses to communicate with Hazel and Augustus directly over the internet, saying he is afraid of the stealing and illegal sharing that prospers there. However, technology in general is shown in a very positive light in the book. Both Hazel and Augustus are being kept alive by new technology: Hazel’s oxygen tank and BiPAP, and Augustus’s prosthetic leg. Furthermore, there is a lot of tastefully written use of modern communication in the book - texting, calling, and email - that allow people to communicate effectively and even saves lives (as when Augustus calls Hazel from the gas station). The book, written for teens born when computers were already becoming ubiquitous, necessarily upholds teenage values of technology as a positive force in medicine, science, and life.
