I have to say it's romantic just because of the intimacy of it all. It's setting alone is really romantic--a field full of beautiful flowers, no one around for miles, nothing but the deep sky above them, only each other to be with--how much more romantic could it get?
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Yeah, I think the main thing that makes it romantic is the idea that they're basically transcending death to be with each other. That and the fact that it's just the two of them. Cloud didn't say "Hey everybody, let's go on a picnic to hang out with Aerith." Instead, he just wanted to be with her by himself. It's like a date between the two of them, instead of a group party. (Plus, nobody else could see her, but you get what I mean)
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Even if it wasn't a flower field, if it's a place where the two can call "Our place", then it's romantic. That's what I think. Something like how in FFIX whenever Garnet sings Zidane keeps saying it's "their song". If you can share something special and intimate with someone you love, then it's romantic.
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Well, let's see. Squall and Rinoa seem to like flowerfields, and Rinoa's not even a floral entrepeneur.
And they promise to meet in one.
So, if Aerith, who has a much stronger association of flowers meets with Cloud in a flowerfield...she's probably not asking how Barret's oil expidition went.
And he's probably not telling her about the last delivery he made.