I have trouble believing this one. Poole has stated that the controversies 4chan was involved in in September had a big negative personal impact, principally dealing with legal threats from celebrities who had their photos leaked on the site. Two other events, which he conspicuously hasn't commented on would have unquestionably left scars on his ego and soul: the 8chan exoduses and 'cuckgate'. I would speculate that these had a far greater negative personal impact on the poor guy than any of the others.
Some of the best original content creators and experienced posters migrated to 8ch, taking the mantle from 4chan as the authentic core of internet culture. It's evident that at least a couple of years ago, moot took some pride in 4chan's leading place in contemporary culture, so to lose the one single point of pride associated with the site must have been a big blow. In his farewell livestream, he responded to repeated questions about 8chan by talking about just about every other 'chan'-style site on the internet, making the omission very obvious, and indicating it was something he took personally.
Second is the situation with one of his friends with whom he was very clearly in unrequited love being revealed and mocked by his own board, giving rise to the current ubiquity of the 'cuck' meme across the internet (and even spreading into mainstream American politics). I know if I had been in his place, this second event would have been quite devastating not only for destroying my hopes and illusions of love and connection with this girl (she even joined in the mockery in a way), but also for having it be that basis of my ignominious fall from beloved benevolent dictator to a hated object of mockery and derision across the entire internet.
When you take these last two events into consideration, I don't think anyone could blame him for his decision to quit. Hope he has a much better time in his future endeavors.