What's the significance of this passage from “The Goblet of Fire”

Question

For a fleeting instant, Harry thought he saw a gleam of something like triumph in Dumbledore's eyes. But next second Harry was sure he had imagined it, for when Dumbledore had returned to his seat behind the desk, he looked as old and weary as Harry had ever seen him.

This happens right after...

Dumbledore questions Harry about the events in the graveyard and the coming back of the Dark Lord.

Why would Dumbledore display a "gleam of triumph" in that specific moment? Is there any references in canon explaining what was going on in Dumbledore's mind?

Answer 1

The answer is in the King's Cross chapter of Deathly Hallows. Therein, Dumbledore explains (or leads Harry to the understanding) that when Voldemort took his blood to regenerate himself, he tied Harry to life; while Voldemort still lived, Lily's protection lived on in Voldemort, tying Harry to life and this is why he didn't die when Voldemort "killed" him.

‘But if Voldemort used the Killing Curse,’ Harry started again, ‘and nobody died for me this time – how can I be alive?’
‘I think you know,’ said Dumbledore. ‘Think back. Remember what he did, in his ignorance, in his greed and his cruelty.’
...
‘He took my blood,’ said Harry.
‘Precisely!’ said Dumbledore. ‘He took your blood and rebuilt his living body with it! Your blood in his veins, Harry, Lily’s protection inside both of you! He tethered you to life while he lives!’
‘I live ... while he lives? But I thought ... I thought it was the other way round! I thought we both had to die? Or is it the same thing?’