Knowing their personality and daily interaction, this confession is really suit those two. Some of you said there's no build up, that's true. But I think this confession just like formality for them, that's why Natsu said "business as always" because before that, they already interacting like couple. Well, that should a build up at some point because now they already know they like each other, but it still make sense there's no build up right after the confession due to their personality and daily interaction.