When Snape's changed his allegiance from Voldemort to Dumbledore he did so expressly because he wanted to honour Lily's memory by protecting Harry. It was Dumbledore himself who persuaded him to take this course of action.
“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.”
(Deathly Hallows, Chapter 33, The Prince's Tale).
If this was the basis on which Snape changed sides and the basis for Dumbledore's trust of Snape then why on earth would he be surprised, 15 years later, that Snape's motivations remain the same? Dumbledore seems genuinely surprised that Snape is still acting out of his love for Lily. He is confident that telling Snape that Harry must die won't cause him disquiet. He questions why Snape's motivations would be unchanged "after all this time".
[Snape] stood up. “You have used me.”
“Meaning?”
“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.
(Deathly Hallows, Chapter 33, The Prince's Tale).
Bear in mind both that:
- Dumbledore was completely confident in Snape's loyalty. He trusted him implicitly and absolutely.
- The sole basis for Snape's loyalty, based on what Snape actually revealed to Dumbledore, was his commitment to protect Harry on Lily's behalf.
Why does Dumbledore suddenly cast doubt on Snape's commitment to honouring Lily and protecting Harry here? Surely that was the basis for his allegiance to the Order all along? Doesn't Dumbledore's absolute trust of Snape imply that he was completely confident that Snape's motivations were unchanged?
