Why must Harry Potter be told that a part of Voldemort's soul latched itself onto the only living thing it could find, again?

Question

In Severus's memory, he and Dumbledore have a talk about Harry Potter.

Professor Albus Dumbledore: There will come a time when Harry Potter must be told something. But you must wait until Voldemort is at his most vulnerable.

Professor Severus Snape: Must be told what?

Professor Albus Dumbledore: On the night Lord Voldemort went to Godric's Hollow to kill Harry, and Lily Potter cast herself between them, the curse rebounded. When that happened, a part of Voldemort's soul lached itself onto the only living thing it could find. Harry himself. There's a reason Harry can speak with snakes. There's a reason he can look into Lord Voldemort's mind. A part of Voldemort lives inside him.

This quote was found on IMDB, and I do believe it is the movie edition. I also believe that this happened in the books as well. (If not, and edit would be much appreciated)

In Harry Potter and the Chamber of Secrets (book), Dumbledore has already told Harry that a part of Voldemort was in Harry.

Dumbledore: Unless I'm much mistaken, he transferred some of his own powers to you the night he gave you that scar. Not something he intended to do, I'm sure...

Harry: Voldemort put a bit of himself in me?

Dumbledore: It certainly seems so.

Harry was already told that some of Voldemort was in himself. Why did Dumbledore ask Severus to tell him again? Was is because he added that Harry must die?

Why must Harry Potter be told that a part of Voldemort's soul latched itself onto the only living thing it could find, again?

Answer 1

Because Harry needed to realize that he was specifically a last "Horcrux" (as Dumbledore called it), and realize that (1) he needed to be killed by Voldemort - (2) AND by his own choice - to destroy that last soul piece AND protect others from Voldemort.

"a bit of himself in me?" doesn't carry such implications or imperatives - it merely connects Harry to Voldemort, and he successfully learned to block that connection after Dobby's death.

Answer 2

Harry needed to understand why it was imperative that he himself had to die in order to fully destroy Voldemort and ensure Voldemort's death in turn. That little piece of soul had to not live as it had been (I hesitate to say "die" here because it was the last piece of Voldemort's soul that became the mangled, baby-like creature at King's Cross -- it was neither alive, nor dead, but in limbo.).

Why he had to be told specifically that Voldemort's soul latched itself onto the only living thing in the room, unfortunately canon doesn't answer this question. Perhaps J.K. Rowling has, though. If I can find a quote I will edit it in.

ETA: It's important to remember that Harry is NOT a Horcrux, despite what Dumbledore says.

JKR addresses whether or not Harry is a proper Horcrux:

"Here is the thing: for convenience, I had Dumbledore say to Harry, "You were the Horcrux he never meant to make," but I think, by definition, a Horcrux has to be made intentionally. So because Voldemort never went through the grotesque process that I imagine creates a Horcrux with Harry, (SU: Mm-hm.) it was just that he had destabilized his soul so much that it split when he was hit by the backfiring curse. And so this part of it flies off, and attaches to the only living thing in the room. A part of it flees in the very-close-to-death limbo state that Voldemort then goes on and exists in. I suppose it's very close to being a Horcrux, but Harry did not become an evil object. He didn't have curses upon him that the other Horcruxes had. He himself was not contaminated by carrying this bit of parasitic soul."

[SNIP]

"I do think that the strict definition of "Horcrux," once I write the [Harry Potter Encyclopedia], will have to be given, and that the definition will be that a receptacle is prepared by Dark Magic to become the receptacle of a fragmented piece of soul, and that that piece of soul was deliberately detached from the master soul to act as a future safeguard, or anchor, to life, and a safeguard against death." JK Rowling - Pottercast 12.23.07 - The Leaky Cauldron

So, yes, the hat detected the portion of Voldemort's soul within Harry. But, no, Harry was not an official Horcrux.