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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?