Well, it's tough to make blanket statements about a whole species. Maybe aggression's just more common.

[Or it could be a matter of nature against nurture. If people use aggression to keep them in line, why wouldn't they go nuts?

He doesn't voice that opinion for now, but it's definitely crossed his mind.]

But I think as long as you keep consistent with the rewards, it'll help sooner or later. I think most Pokémon understand each other, but if you wanted to play it safe, you could always grab another dragon type.