The line ‘I don’t hate you too’ between Mai and Zuko is actually something that originated between them back in their adolescence, when they were finally admitting to having small crushes on each other (and doing their best to keep that official fact hidden from Azula). Ironically the first time the double meaning phrase was used was two days before the war meeting that ended in the Agni Kai and Zuko’s subsequent banishment. Neither of them would get to say it to each other again for another three years when Zuko returned home upon 'slaying the Avatar’. It has since become something of a private joke between them now that they’re officially dating once more.