When Hazel and Augustus fall in love on the plane.
Augustus: “I'm in love with you. I am. I'm in love with you, and I'm not in the business of denying myself the simple pleasure of saying true things. I'm in love with you, and I know that love is just a shout into the void, and that oblivion is inevitable, and that we're all doomed and that there will come a day when all our labor has been returned to dust, and I know the sun will swallow the only earth we'll ever have, and I am in love with you.”
John Green's gracious and understanding response to the accidental early leaks:
"Mistakes happen. The people who made this error were not bad or incompetent people, and they were not acting maliciously. We all make mistakes, and it is not my wish to see Barnes & Noble or any of their employees vilified."
The fact that no one who received the book early revealed any spoilers.