The reading of which gave me an idea: Re-reading this book in light of Rowling revealed that Dumbledore is gay, I have a theory: there was more to Dumbledore/Grindlewald than mere friendship. I think they were lovers, and the death of Dumbledore's sister somehow caused a falling-out between the two, maybe in combo with some moral dilemma Dumbledore was going through. And Dumbledore's 5 year reluctance to finally challenge Grindlewald was due to lingering affection for the man. Here, have some evidence:
In the letter from Dumbledore to Grindlewald wherein he is talking about "Wizard dominance being for the muggles' own good," he ends the letter thusly (bold is my own emphasis): We seize control FOR THE GREATER GOOD. And from this it follows that where we meet resistance, we must use only the force that is necessary and no more. (This was your mistake at Durmstrang! But I do not complain, because if you had not been expelled, we would never have met. Doesn't that sound like "lovers" to you? It does to me. Shaky evidence, I know, but that's the only thing concrete I have, the rest is just a feeling, an overall pattern emerging from bits and pieces.