“But this is touching, Severus,” said Dumbledore seriously. “Have you grown to care for the boy, after all?”
“For him?” shouted Snape. “Expecto Patronum!
From the tip of his wand burst the silver doe: She landed on the office floor, bounded once across the office, and soared out of the window.

This exchange appears in Chapter Thirty-Three, though it takes place before the events of this novel. This is one of Snape’s memories that Harry views through the Pensieve after Snape’s death. In it, Dumbledore has just revealed that he has known for a long time that Harry will have to die before Voldemort can be killed. Snape accuses Dumbledore of lying to Snape and using him, because Dumbledore had told him years before that he could protect Lily Potter’s son (Harry), which is the sole reason Snape has devoted himself to this dangerous project. In these lines, Snape makes it clear that his love for Lily Potter has always been his motivation, demonstrating this fact in the dramatic visual image of his own Patronus, which takes the same form that Lily’s did—that of a doe.

Dumbledore’s words are wonderfully ambiguous. When Harry sees this conversation, he is learning for the first time that he has to die, and that Dumbledore knew this and deliberately kept it from him. Dumbledore’s words may or may not be sarcastic, may or may not suggest that Dumbledore does not care for Harry, may or may not imply that Dumbledore has been using Harry.