Why did The Gentlemen need 7 hearts? I can't remember if they explained it and it's driving me nuts!

i don't know exactly. But in the episode there were only six gentlemen and i'm guessing they needed seven hearts to maybe bring up the master gentleman? since they were only six maybe they had a leader или an all powerful gentleman that could only be summoned by collecting seven human hearts maybe.

Actually, the reason why The Gentlemen need 7 hearts is never fully explained on Btvs. According to "Hush"'s script, they need 7 hearts to 'survive', but during the episode things are never further developed on this argument. We can assume that, in regard to the Gentlemen's nature originating from a fairytale, this can be reconnected to some gloomy theme, like they need the hearts for human consumption, to regenerate themselves and\or to break some curse they suffered from humans. But all of those are speculations, and to be honest only Mr.Joss Whedon himself could properly answer to your question.

why do the doctorsneedhearts