Professor McGonagall's Entire Backstory Explained

As it turned out, Dumbledore and Minerva had much more in common than she ever could have anticipated. Many readers know, in the aftermath of the Potter books, that Dumbledore was actually in love with Gellert Grindelwald, the dark wizard who was his friend as a teenager. In the end, Dumbledore and Grindelwald faced off during the first Wizarding War, and Dumbledore defeated him, breaking his own heart in the process. For that reason, when Minerva found out that Dougal — whose letters she saved for years — had married a farm girl in her hometown, she turned to Dumbledore to help soothe her freshly broken heart.

After finding Minerva distraught after hours, Dumbledore proved a great source of comfort for the heartbroken witch, confiding in her about his family tragedy and his own lost love. From then on, the two shared an extremely close yet platonic friendship, based on mutual respect and total confidence between them, which lasted until Dumbledore's death during Harry Potter's sixth year at school.