Epigraphs are like little appetizers to the great entrée of a story. They illuminate important aspects of the story, and they get us headed in the right direction.
As the tide washed in, the Dutch Tulip Man faced the ocean:
"Conjoiner rejoinder poisoner concealer revelator. Look at it, rising up and rising down, taking everything with it."
"What's that?" I asked.
"Water," the Dutchman said. "Well, and time."
– PETER VAN HOUTEN, An Imperial Affliction
The epigraph is an imaginary quote from an imaginary book called An Imperial Affliction. But who said the imaginary wasn't worth thinking about?
Time is a big freakin' deal in The Fault in Our Stars because Hazel and Augustus don't have a lot of it left. The two are always discussing infinities and how some people have more time than others. Hazel also references the fact that eventually, all that we know will be gone and fade into oblivion. Sounds a lot like the epigraph, right?
Looks like the epigraph has done its job to set the tone.