Well, I searched around the Internet and found some interesting stuff.
So in the general folk lore there were these dragons, like Fire Dragons, Water Dragons, and Thunder Dragons. So I guess it kinda makes sense that they would resist themselves.
Also, I guess Gamefreak wanted dragon types to be superior to others, so they made dragons resist common types. (Wild guess on my part, but I guess they considered electric to be common because of Pikachu)
So yeah. Sorry if that wasn't the answer you were looking for, but it's the best I can do :)
