Hazel's dad comforting her towards the end of the book after Augustus' death.
"'I don't think defeatism is honest,' Dad answered, 'I refuse to accept that.'" Hazel's Dad really stands out in a book full of bitter cynics and desperate idealists. He's some sort of happy medium. Heck, almost every word out of his mouth is heartwarming, be it comforting Hazel, defending her mother, or just providing some tension-breaking comic relief.
This line from Augustus:
Augustus: Oh, I wouldn't mind, Hazel Grace. It would be a privilege to have my heart broken by you.
Issac's Eulogy: "When the scientists of the future show up at my house with robot eyes and they tell me to try them on, I will tell the scientists to screw off, because I do not want to see a world without him."
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.