“You are protected, in short, by your ability to love!” said Dumbledore loudly.  “The only protection that can possibly work against the lure of power like Voldemort’s!  In spite of all the temptation you have endured, all the suffering, you remain pure of heart, just as pure as you were at the age of eleven, when you stared into a mirror that reflected your heart’s desire, and it showed you only the way to thwart Lord Voldemort, and not immortality or riches.  Harry, have you any idea how few wizards could have seen what you saw in that mirror?  Voldemort should have known then what he was dealing with, but he did not!

“But he knows it now.  You have flitted into Lord Voldemort’s mind without damage to yourself, but he cannot possess you without enduring mortal agony, as he discovered in the Ministry.  I do not think he understands why, Harry, but then, he was in such a hurry to mutilate his own soul, he never paused to understand the incomparable power of a soul that is untarnished and whole.”

“But, sir,” said harry, making valiant efforts not to sound argumentative, “it all comes to the same thing, doesn’t it?  I’ve got to try and kill him, or —”
“Got to?” said Dumbledore.  “Of course you’ve got to!  “But not because of the prophecy!  Because you, yourself, will never rest until you’ve tried!  We both know it!  Imagine, please, just for a moment, that you had never heard that prophecy!  How would you feel about Voldemort now?  Think!”

Harry watched Dumbledore striding up and down in front of him, and thought.  He thought of his mother, his father, and Sirius.  He thought of Cedric Diggory.  He thought of all the terrible deeds he knew Lord Voldemort had done.  A flame seemed to leap inside his chest, searing his throat.

“I’d want him finished,” said Harry quietly.  “And I’d want to do it.”

“Of course you would!” cried Dumbledore.  “You see, the prophecy does not mean you have to do anything!  But the prophecy caused Lord Voldemort to mark you as his equal. . . . In other words, you are free to choose your way, quite free to turn your back on the prophecy!  But Voldemort continues to set store by the prophecy.  He will continue to hunt you . . . which makes it certain, really, that —”

“That one of us is going to end up killing the other,” said Harry.  “Yes.”

But he understood at last what Dumbledore had been trying to tell him.  It was, he thought, the difference between being dragged into the arena to face a battle to the death and walking into the arena with your head held high.  Some people, perhaps, would say there was little to choose between the two ways, but Dumbledore knew — and so do I, thought Harry, with a rush of fierce pride, and so did my parents— that there was all the difference in the world.

Harry Potter and the Half-Blood Prince – J.K. Rowling