Albus Dumbledore's "deepest and most desperate desire" has long mystified Potter fans. The Mirror of Erised, first introduced in Harry Potter and the Philosopher's Stone, supposedly reveals one's greatest desire and yet, when asked, Professor Dumbledore claims to have seen himself holding "a pair of thick, white woolen socks." Obviously, Dumbledore was lying, so what did he actually see?
Fans have long hypothesized that "woolen socks" was a metaphor for family. Dumbledore loved to knit, a hobby he likely picked up from his mother Kendra and younger sister Ariana. This implies he probably saw his family, alive and well, in the mirror, a theory author J.K. Rowling would later confirm during a webchat with Bloomsbury Publishing. Unfortunately, that wasn't always the case. The latest trailer for Fantastic Beasts 2 just revealed what Dumbledore saw in the mirror in his younger years, and it was neither family nor knitting patterns, or even his dearly departed sister Ariana. It was his friend, Gellert Grindelwald.
Grindelwald (played in the films by Johnny Depp) was the Dark Wizard of the early 1900s. He was also Dumbledore's friend and one true love, a Potter fact Rowling would later confirm upon revealing that Dumbledore was gay. Obviously, Dumbledore's feelings were unrequited (Grindelwald was too busy thinking about Muggle domination to bother with romance), but aside from wanting to be with his friend, what other reasons could there be for Gellert Grindelwald showing up in the mirror? How could a murderous psychopath, who was probably responsible for Ariana's death, possibly prove more desirable for Dumbledore at this period in his life? Turns out, there's several reasons.
A Broken Friendship
Albus Dumbledore was a man of his word who rarely gave up on friends. Gellert Grindelwald was his greatest and most cherished friend -- the one person he probably regretted losing. Even through Ariana's death, and despite losing his brother's trust, Dumbledore refused to give up on him. He had hoped to peacefully resolve their differences and put the past behind them, but it was easier said than done. Their paths and ideals (in that era) couldn't be more different, and we all know how that ended. Dumbledore defeated his friend in what was then the greatest battle the Wizarding World has ever known, leaving Grindelwald with little to look forward to: Lifetime incarceration and death.
This might be why Grindelwald appeared in the Mirror of Erised. Dumbledore desperately wanted to make things right and live in a world where he could peacefully coexist with Grindelwald -- whether as friends or partners -- but even he knew it wasn't his choice to make. And the truth broke his heart.