"Her son lives. He has her eyes, precisely her eyes. You do remember the shape and color of Lily Evan's eyes, I am sure?"
"DON'T!" bellowed Snape. "Gone...Dead..."
"Is this remorse, Severus?"
"I wish...I wish I were dead..."
"And what use would that be to anyone?" said Dumbledore coldly. "If you loved Lily Evans, if you truly loved her, then your way forward is clear."
Snape seemed to peer through a haze of pain, and Dumbledore's words appeared to take a long time to reach him.
"What- what do you mean?"
"You know how and why she died. make sure it was not in vain. Help me protect Lily's son."
"He does not need protection. The Dark Lord has gone-"
"The Dark Lord will return, and Harry Potter will be in terrible danger when he does."
There was a long pause, and slowly Snape regained control of himself, mastered his own breathing. At last he said, "Very well. Very well. But never- never tell, Dumbledore! This must be between us! Swear it! I cannot bear- especially Potter's son...I want your word!"
"My word, Severus, that I shall never reveal the best of you?" Dumbledore sighed, looking down into Snape's ferocious, anguished face. "If you insist..."
When I read this part for the first time I literally stared at the page with my jaw dropped for like a whole minute, then I took a look back through the entire series restringing it together with my newly acquired knowledge.
--
"I have spied for you and lied for you, put myself in mortal danger for you. Everything was supposed to be to keep Lily Potter's son safe. Now you tell me you have been raising him like a pig for slaughter-"
"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. Dumbledore watched her fly away, and as her silvery glow faded he turned back to Snape, and his eyes were full of tears.
"After all this time?"
"Always," said Snape.