The title The Fault in Our Stars is taken from Shakespeare's Julius Caesar. The nobleman Cassius says,
The fault, dear Brutus, is not in our stars, but in ourselves that we are underlings.
The question of whether or not we determine our own fates, or if everything is predetermined (or "written in the stars"), is as old as human experience itself. For Augustus and Hazel, the title embodies the challenges presented to them by fate (their respective cancers) and how they choose to respond.
As Augustus writes to Hazel before his passing,
Getting hurt in this world is inevitable, but we do get to choose who we allow to hurt us.
In other words, cancer may have been written in their stars, but the choices they make with the time they have is theirs alone to make. Both characters know the heartache they risk in falling in love with one another. In the end, for both Augustus and Hazel, those choices prove to be happy ones.