What is the mood in John Green's The Fault in our Stars?

The Fault in Our Stars, written by John Green, tells the story of two teenagers who meet at their cancer support group. Sixteen-year-old Hazel has had thyroid cancer for several years and has come to terms with her mortality and her place in the world. Hazel is quite mature for accepting her fate, but her mother also thinks that she has become depressed. (Hazel thinks depression is a side-effect of knowing you will die.) One day when Hazel is attending support group, she meets seventeen-year-old Augustus, who is recovering from osteosarcoma. She thinks he is pretentious and a little obnoxious, but funny and charming at the same time. Hazel and Augustus forge a bittersweet friendship after Augustus pretends to smoke a cigarette-- he tells Hazel that he likes the metaphor of holding the killing-thing (the cigarette) but not giving it the power to kill you (by lighting it) as a means of having some power in his own mortality. 

From the very beginning, we can sense that this story is bittersweet. It is depressing to think or know that someone will die, to know that we, ourselves, will die. For that reason, the mood or tone of this book is often sorrowful or full of grief.

Hazel and Augustus have two different approaches to reconciling the knowledge of their morality. For Hazel, she is comforted by knowing that everyone will die someday. Augustus does not appreciate that sentiment as much, but feels that he can at least make the most out of his time while he is still alive. Augustus wants to make the most out of his, and Hazel's, time as friends by offering to use his "wish," granted by an organization for children and teens with cancer, to take Hazel to meet her favorite author. When the two travel to Amsterdam and enjoy the beautiful city, a fancy dinner, and meet this author, the story is really hopeful. Hazel and Augustus know that they have cancer and decide to do this ambitious and happy thing not as a way of denying their health status but in spite of it. They want to show cancer who's boss! 

Sadly, their trip is not all they hoped for. When they meet Hazel's favorite author to ask him their pressing questions, they find out he is a miserable drunk. Hazel and Augustus storm out, disappointed that he was not who they expected, but proud of themselves for accomplishing what they had set out to do. What's more, they had grown closer to each other during the journey and fallen in love. 

All throughout the book, sorrow, grief, and joy are interwoven to create a heart-wrenching and very real narrative. Life is never made up of only happy things or only terrible things, and John Green has done an excellent job of showing this. It is quite sobering. Even when Augustus dies from a relapse of cancer, Hazel cherishes his memory and that which he taught her about how to let go and make the most of the time she has.