In Melanie Koss and William Teale’s “What’s Happening in YA Literature? Trends in Books for Adolescents” the authors conduct many studies about what makes YA reading so popular. When analyzing popular subject matter, they found that overtime, YA books have been called “problem novels”—“novels with social issues that affect teens..—books that deal with a significant even in a teenagers life that transforms one from childhood/adolescence to adulthood.” (Koss/Teale 567) The author writes that YA books have the reputation of being edgy and filled with touchy social issues like sex and violence. And although she listed books that do have these issues, the most popular subjects were about fitting in and finding ones self. When conducting a poll on subject matter, 85% of the books where subjected “finding themselves” (567) while only 32% were on relationships and 25% on mental illness; (567) which are two main themes in TFIOS. As both Hazel battles with depression due to her cancer, but then finds love with another cancer kid who gets her out of her depression and finds herself. When looking at the whole chart, TFIOS touches upon most of the topics. Which is why it is represented as such a popular YA read.
Not a ton of analysis here – it will be worth your while to find a source that gives you more to say.