Dumbledore is a pre-eminent magician, magical "scientist", and holds tons of other posts (Chief Warlock of Wizengamot, Supreme Mugwamp, Headmaster of Hogwarts, Leader of the Order of the Phoenix, Best Dogcatcher of Hogsmead).
So why in the world would he devote a not-inconsiderable time to annotating a children's book? (Tales of Beadle the Bard)?
If it was ONLY about the Tale of Three Brothers, we could blame it on fascination with the Hallows.
But he annotated ALL the stories, so it clearly wasn't about the Hallows.
Canon based answers only, please