Why did Dumbledore have a look of triumph?

[robin_mcdonagh at hotmail.com]

No: HPFGUIDX 27226

At the end of book four, Harry Potter and the Goblet of Fire, as 
Harry explains that Wormtail took some of his blood, put it in the 
cauldron so that Voldemort could touch him again, Dumbledore looks up 
with a look like truimph, which Harry says is quickly gone.
I'm not sure where, but I'm sure that I saw on a very reliable site a 
copy of an interview, where someone asks that very question. J.K. 
said that she won't tell why, but she is sure that someone will guess.
Is it obvious to anyone? Not to me. Please, if anyone has any ideas, 
it would be appreciated if you could post your ideas. I know that 
there are more pressing questions, but this one really has me 
stumped, with hardly a guess as to why he could have had this 
reaction.
If you would like to email me directly, I am at 
robin_mcdonagh at hotmail.com.
Thank you for reading my ever-so-interesting bullitin about nothing.