>>96178470
>Wasn't it Arianna's death? Or was the chasm already present in the Albus-Abelforth-Gellert fight?
Based on his later actions, I think it was being rescued by Muggles from his burning house. He'd been skewing Dark because of how Muggle kids abused his sister, right up until the fire. Then Muggles saved his life, and presto! Dumbledore's suddenly all about second chances.