"Falling in love can blind us to an extent," Rowling explained Friday in front of a packed house at New York's Carnegie Hall, where she capped off her first U.S. book tour since 2000.
Which explains why the brilliant wizard was briefly blinded as a young man by the charm and skill of Gellert Grindelwald, his companion turned archnemesis who turned out to be more interested in the Dark Arts than a three-bedroom craftsman in Hogsmeade.
After Dumbledore was "horribly, terribly let down," Rowling explained, he went on to destroy Grindelwald in what is considered in the wizarding world to have been the ultimate wand-toting battle between good and evil.
That love, she said to raucous applause, was Dumbledore's "great tragedy."
"If I had know this would have made you so happy, I would have told you years ago," Rowling said.
While working on the sixth Potter film, Harry Potter and the Half-Blood Prince, which focuses largely on Dumbledore and Harry's relationship, as well as the elder wizard's interaction with a young Voldemort, Rowling said that she slipped director David Yates an eye-opening note after noticing that there was a reference in the script to a girl in Dumbledore's past.