In the fourth book when Dumbledore realizes that Voldemort has come back...?

At the end of the fourth book when Dumbledore is told that Voldemort had been reborn why is there a look of triumph in his eyes? Was this ever explained in the book? I mean I read all the books but I don't remember them ever explaining that one.

In the fourth book when Dumbledore realizes that Voldemort has come back...?virus scanner

Dumbledore explains it in the "King's Cross" chapter when Harry is semi-dead. Chapter 35, p. 708 (US hardback), Dumbledore tells Harry to "Think back..." Harry answers (next page), "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."

On p. 710, Dumbledore adds:

"He took your blood believing it would stregthen him. He took into his body a tiny part of the enchantment your mother laid upon you when she died for you. His body keeps her sacrifice alive, and while that enchantment survives, so do you and so does Voldemort's one last hope for himself."

Harry asks:

" And you knew this? You knew -- all along?"

Dumbledore answers:

"I guessed. But my guesses have usually been good."

So Dumbledore was doing his guessing at the moment his eyes gleamed with a look of triumph. Realize that this look of triumph is what redeems Dumbledore from being a cold-hearted Machiavellian bastard who foresaw the need for Harry to die to get rid of the piece of Voldemort's soul inside of him. This look says that he not only wanted to defeat Voldemort, but to save Harry. It is in this King's Cross exchange with Dumbledore that Harry also realizes what that look of triumph meant - that Dumbledore cared for him and not just "the greater good." But Harry walked to his death only seeing the "greater good" part without any hope for himself. He has good reason to be angry with Dumbledore, but he's not, proving that he really is the better man.