So, Dumbledore is obviously a divisive character. It’s pretty well accepted that JK likes him and thinks of him as a good guy (though she also thinks that way about Snape, which, whatever), but a lot of the fandom seems to think he’s a pretty terrible character. Ruthless, manipulative, the whole nine. I’ve been pretty neutral on him, actually – I really like his whole befuddled Gandalf thing, and I think he truly does want what’s best for the wizarding world. But I think that part of the reason that people recoil from him so strongly during re-reads and re-watches is that he is so implicitly trusted by so many characters, and in a lot of ways he doesn’t seem worthy of that trust.
Sirius Black is my F A V O R I T E character, you guys. I love him so much. I want to snuggle with him while eating ice cream and watching eighties movies and make everything in his life better. After being disowned by his family for standing up against their beliefs, he was accused of betraying his best friend because he shared those beliefs, and spend over a decade of his life in Azkaban for that crime (without a trial, thanks Barty!) (in case it’s unclear, he was innocent).
Dumbledore was on the Wizengamot at the time – he is mentioned throughout the series as being a powerful and influential wizard – and yet he (apparentely, from what we see) does nothing to prevent this from happening, makes no moves whatsoever to suggest that possibly there was another explanation, and – again, from what we see – seems to accept Sirius’ guilt wholeheartedly. If I were Sirius, I’d be PRETTY MAD that someone who had known me since childhood and worked with me completely accepted that I was a liar and would happily sell out my best friend.
Sirius, however, seems to hold no grudges. Throughout the series he seems to have the same level of implicit trust in Dumbledore that everyone else does, to the point that it’s almost fucking creepy. I could possibly accept that Dumbledore was simply distracted or had been hurt too many times while Sirius was being locked up – after all, his best friend turned out to the the precursor to Voldemort and the war was still being fought, though the big guy was out of the picture – but I can’t imagine that drama-queen-numero-uno Sirius Orion Black would forgive and forget quite so easily. At least not without some VERY DRAMATIC scene / argument – which we never saw him getting.