Responses
Part of it is in ch. 2, basically he had a crush on this particular workmate and just when that one happily announced that he'll soon marry his girlfriend, Natsumes friend (cousin? I don't remember) proposed swiitching over to his company. Naturally Natsume couldn't ever explain this abysmally bad timing (right before they finished the project - can't be any worse), hence the animosity.
Wait is there a back story on why Natsume quit his company and started working for the other company