I'm not hating on this ship(if u ship them, that's fine to me), but I just don't understand where the idea of the being a couple came from. I mean, they haven't said anything to each other in the show, and we don't know anything about Comet Tail other than his name and what he looks like...

3475407 I suppose the ship came from the one scene in that episode where Applejack left Ponyville. During the mayor's speech they are seen standing together. It was only for five seconds or so, but that was how long it took for Derpy to spawn.