The Pure of Heart

This may not actually make a whole lot of sense to people unfamiliar with the series, but, then again, themes of good and evil are universal. 

“It is Voldemort’s fault you were able to see into his thoughts, his ambitions, that you even understand the snakelike language in which he gives his orders, and yet, Harry, despite your privileged insight into Voldemort’s world, you have never been seduced by the Dark Arts, never, even for a second, shown the slightest desire to become one of his followers!”

“Of course I haven’t!” said Harry indignantly. “He killed my mum and dad!”

“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.”

— Harry Potter and the Half-Blood Prince, 511