Still, that doesn't make sense. If an Eeveelution was, by canon, shy or didn't like attention, they'd probably show it during the short as being shy or not liking attention. And I don't think GameFreak would want to make your Pokemon dislike you just so that you can get a variant of an evolution. That seems rather harsh, and I doubt they'd do it for the Dragon-type since they aren't even considered bad or evil (neither is the Dark-type).
Like another user posted, tradition has been broken by GameFreak over the past couple of years, so I doubt we'll see another Eeveelution this Gen. Besides, having only one Eeveelution this Gen makes up for GenI's introduction of three.
1. If it was a personality thing, it's a good chance that none of the Eevees in the short had the disposition to become the Dragon Eeveelution. Therefore, it wouldn't have shown up.
2. Sylveon seems like it was used more to showcase the new type, rather than just being an Eeveelution. That's why they made a big deal of not telling us the type.
3. They keep using Sylveon in the Pokemon-Amie feature, which means this is possibly the evolution method, considering it probably isn't a known method of evolution and it isn't a Mega. If they were to introduce another one, it would have to be the same evolution method. Since anger/love are the only two reactions we've been shown (the anger point on Pikachu and hearts over Lucario), it's a logical conclusion that that would be the deciding factor between the two. It could also be seen as not disliking you specifically, but disliking the attention it gets. Because it doesn't like attention.
I think we could get another one, but it might not be shown before the game is released. It would be something special to be found by the people who play with Pokemon-Amie a lot right at the beginning, try different things with Eevee, and hit the right trigger to get the Dragon-type evo. Just because they CAN break tradition and HAVE BEEN doing so, doesn't mean they WILL do it in this case.
Also, technically they released four Eevees, since Eevee is one of them, in gen 1. So we're on an odd number of Pokemon in the evolutionary line anyway with the addition of Sylveon and no partner.