Unsurprisingly for a novel about kids dying of cancer, suffering is a prominent part of the character’s lives. Hazel, Augustus, and Isaac all endure quite a bit of physical and emotional pain. The buildup of fluid in Hazel’s lungs deprives her of oxygen, leading to a bout of intense pain that lands her in the emergency room. Isaac has to contend with losing his remaining eye, which leaves him blind and leads his girlfriend to break up with him. Augustus physically deteriorates to the point that he has to take pain medication strong enough to leave him nearly incoherent, and he suffers to know he’ll never accomplish any of the heroic things he wanted to do in his life. In the eyes of the novels’s characters, specifically Hazel and Augustus, all these types of pain are simply a part of living, a side effect of it as Hazel might put it. That doesn’t mean they’re desirable, just that they’re inevitable.
But the most thematically significant type of pain in the novel is that caused by the death of a loved one, and it’s this variety that the novel suggests is the most necessary. Hazel worries a great deal about inflicting this kind of suffering on those around her when she dies, leading her to come up with the metaphor of the grenade that explodes and injures everyone nearby. It turns out she becomes the victim of this kind of pain when Augustus begins to weaken and finally succumbs to his cancer. What Hazel comes to understand is that this type of pain can’t be avoided. Since dying is certain and universal, all people will experience it. But as Hazel comes to recognize over the course of the novel, it isn’t necessarily something one should avoid. She wouldn’t take back the love she feels for Augustus for anything, even though that love is the precise cause of her pain. It’s a blessing and a curse, so to speak. The reason, as Augustus suggests in his letter to Van Houten that Hazel reads at the end of the novel, is that the pain you cause others when you die is a mark that you mattered. Augustus says happily that he left his “scar” on Hazel, meaning he hurt her but he also had an effect on her life that she’ll carry with her always. That type of pain, the novel suggests, is necessary, and in fact it’s a part of joy. Hazel touches on this idea in her eulogy for Augustus. The first thing she says to the gathered crowd is that there’s a quote hanging in Augustus’s that always gave the two of them comfort: “Without pain, we couldn’t know joy.”
The main characters in the novel are forced to confront death in a way that the young and healthy aren't. Although everyone will eventually die, as Hazel points out in Support Group, death's immediacy to the terminally ill means they can't avoid considering what comes after death, and the potential that all that's waiting for them is oblivion. It's a very present fear for Hazel and particularly for Augustus, and in fact it's the first thing they share when they meet at Support Group. Augustus, in response to Patrick's question about what he fears, replies right away with “oblivion,” and Hazel, who rarely ever speaks in the group, picks up immediately. She points out that everyone will some day die, which means everything humanity has ever built could all be for naught, and that just as there was a time before organisms experienced consciousness, there will be a time after as well. She says if the thought is disturbing one should just ignore it, but her tone implies that it's something that can't be ignored, at least not forever.
This theme carries throughout the novel. It's what motivates Augustus's desire to perform some heroic act before he dies and validate his significance. He worries that, without doing something dramatic that lives in people's minds after he's gone, he won't have mattered. His significance, like his consciousness, will simply be consumed by oblivion after his death. For Hazel, the fear of oblivion strikes her in a different way. She needs to know that those close to her, and her relationships with them, will carry on after her death. The comment she overheard her mother make that she'll no longer be a mother stays with her for this very reason, and it's also why she fixates on what happens to the characters in An Imperial Affliction after the protagonist, Anna, dies at the novel's close. She focuses on finding out what happens to Anna's mother and the Dutch Tulip Man, and even Sisyphus the Hamster, as a substitute for worrying about what will happen to her own parents after her death. When Van Houten tells her that the characters simply cease to exist the moment the novel ends, she tells him that it's impossible not to imagine a future for them. What she clearly means is that she has to believe that her own parents will continue on once she's gone, and that's why she's so greatly relieved to learn later that her mother has been taking classes to become a social worker.
What the novel ultimately suggests is that one person's death doesn't consign their significance and relationships to oblivion, and that what makes our lives matter are the relationships we form. As Augustus learns, his importance isn't defined by the fact that his life is temporary, because his importance to those around him will carry on. He leaves his “scar” on Hazel, as he puts it in the letter to Van Houten that Hazel reads at the close of the novel. Hazel, via a different route, discovers much the same. Her mother will continue to be her mother. Nothing, not even her death, can change that.
A refrain repeated throughout the novel is that the world is not a wish-granting factory. In other words, the things we want to come true often don't, and reality can be quite different from our fantasies. Numerous examples appear in the story. Isaac's girlfriend, Monica, breaks up with him just before he has his remaining eye removed, and despite his waiting and hoping, he never receives any word from her afterward. Augustus comes to realize that he will never perform some extraordinary feat of heroism. Hazel knows her lungs won't heal, and her death isn't far off. Peter Van Houten isn't so much the open, caring genius Hazel hopes, but a grouchy and malicious drunk. Augustus's story about his middle-school science teacher, Mr. Martinez, sums up the theme. As Augustus and Hazel fly back from Amsterdam, Augustus tells her he sometimes dreamed of living on a cloud, thinking it would be like an inflatable moonwalk machine. But he learned from Mr. Martinez that, at that altitude, the wind blows at one hundred and fifty miles per hour, the temperature is thirty below zero, and there's not enough oxygen for a person to survive. The man, he tells Hazel, specialized in the murder of dreams.