I'm usually more a Natsu/Gray friendshipper, unless I stumble across a fic where their romantic relationship is pictured like this.
Like Gray said, it was so natural, the transition from mere rivals to lovers, so natural in fact we could have easily overlook it but in the same time couldn't. (I don't think I made any sense but still)
Anyway, it was nice to see some romance where both of them are still in character.