what is the fault of your stars?

The book's title, The Fault in Our Stars, comes from a line in Shakespeare's play in Julius Caesar where Cassius says, "The fault, dear Brutus is not in our stars, / But in ourselves, that we are underlings." Cassius means that it's not fate that dooms men, but their own failings. We are left to ponder the two protagoniosts and how much of their pain can be related to fate and how much can be realted to teen angst.