We're going to turn to Big Willy for this one. The book's title, The Fault in Our Stars, comes from a line in Shakespeare's play in Julius Caesar where Caesar says, "The fault, dear Brutus is not in our stars, / But in ourselves, that we are underlings."
Caesar seems to be saying that it's not fate that dooms men, but instead their own failings. That's right: according to this guy, you are to blame for the nastiness in your life.
John Green isn't quite on board. The title The Fault in Our Stars seems to argue that sometimes it's not our fault; sometimes the bad stuff just can't be avoided. Hazel and Augustus sure didn't do anything to cause their cancer and it's a fact that they cannot avoid or change.
But the beauty of the message is that they can still live and make their decisions despite the fault in their stars, even when they know the inevitable fate that awaits them.