It's important to note, first of all, that Albus Dumbledore was loved — by us, and by you — long before Harry Potter and the Deathly Hallows.
He was loved in Philosopher's Stone, when he comforted Harry about the loss of his parents; he was loved in Order of the Phoenix, where he faced off against Lord Voldemort; he was loved in Half-Blood Prince, in which he sacrificed himself for the greater good. If anything, Albus Dumbledore is as much loved as Harry Potter himself — maybe even more so.
And yet, it wasn’t until Deathly Hallows that we realised: did we ever really know Dumbledore? Or had we merely met him? Sure, we knew Dumbledore as well as Harry knew Dumbledore: as a kind, wise, lovely bearded headmaster; as one of the wizarding world's most famous figures; as a wizard of incredible power and prestige. But know him?
To truly know someone, you have to go beyond the mystique of what they are, and find out who they were: the past that shaped them, the experiences and decisions that informed who they have become. And that is exactly what we got in Deathly Hallows — especially the chapter ‘King's Cross’.
It’s one of the final chapters in Deathly Hallows, a book defined not only by the end of the Harry Potter series — and indeed, the end of Lord Voldemort — but by Harry discovering that Albus Dumbledore was not the man that he thought he was.
For with Dumbledore's death came revelations of a murky past; stories of Dark Arts and a mysterious sister. As revealed by his brother Aberforth, Dumbledore used to be a different man. He was young, brilliant and frustrated; his talents were held back by the responsibility of caring for his sister Ariana, who was left traumatised after a Muggle attack. Aberforth bitterly paints Dumbledore as pompous and arrogant, as thinking he was above the task, that it was a waste of his talents. Hence his seduction by 'equal' Gellert Grindelwald, a Dark wizard with plans for a new wizarding order — a world where Muggles were taught their place. Their friendship partly resulted in the death of Ariana, killed during a three-way duel between Albus, Aberforth and Grindelwald.
These revelations troubled Harry; they tainted his perception of a man he long admired. But then ‘King's Cross’ happened. You no doubt know the circumstances: Harry, having found out that he himself was a Horcrux, has sacrificed himself to Lord Voldemort. In 'death', he has woken in a strange but familiar place (King's Cross station) only to be greeted by a strange and familiar friend. It's unclear, of course, whether Dumbledore is actually real, whether this King's Cross station is some kind of purgatory between life and death, or whether it's just some sort of elaborate dream. But as Dumbledore says, ‘Of course it is happening inside your head, Harry, but why on earth should that mean that it is not real?'
Real or not, ‘King's Cross’ allows Harry to confront Dumbledore — or, at the very least, allows Harry to come to terms with Dumbledore's demons. Ashamed, and with tears in his eyes, Dumbledore tells Harry how he had been a selfish young fool; how he — 'trapped and wasted' — had let himself be seduced by the brilliance of Grindelwald and the power of the Deathly Hallows.
'Master of death, Harry, master of Death! Was I better, ultimately, than Voldemort?' Despite Harry's protestations, he goes even further to say that power is his greatest weakness, his greatest temptation, and that he should never be trusted with it; he even reveals that he turned down the post of Minister for Magic several times, for fear that absolute power would corrupt him absolutely. It's a version of Dumbledore that Harry has never seen before — vulnerable, flawed, human.
And that's what's so important about ‘King's Cross’. It's the chapter that humanises Dumbledore, that grounds him from the heights of gods, that lets us truly know him for the first time. And despite the faults, despite Dumbledore perhaps not being the perfect wizard Harry thought he was, never before has Dumbledore seemed more heroic. For men and women are not born great. They learn greatness over time — from experience, from mistakes. Dumbledore looked at his deeds, at his flaws, and he had the wisdom to confront and overcome them; he fought the greatest nemesis there was — himself.
And in the end, that's what made him so remarkable: for in order to become a great wizard, Dumbledore had to know what it meant to be a bad one. Who better to teach the next generation of wizards? Who better to face Lord Voldemort? Who better to send Harry on his way from King's Cross station, with one last piece of wisdom: ‘Do not pity the dead, Harry. Pity the living, and, above all, those who live without love.'