Breaking The Divide

When Vaatu gloats that he broke the divide between the spirit and human worlds, there doesn't really seem to be an exact reason. I combine that with my head canon that he and Raava are literally "broken" yin and yang to the point that they can't function together as they should...but that there was a time that they used to. Since yang, the "light" force is technically masculine, Vaatu used to be the light spirit, while Raava was once the spirit of darkness. The only time there was real balance was while they were partners, but the scales broke when Vaatu opened the divide...for...some reason.

Since the lesser spirits were able to come through after him, I would think that he was trying to do it for them, but he and Raava were the only ones strong enough to handle it, at the expense of their own personal balances. Vaatu came through first, and Raava tried to protect him, but she failed. Their roles shifted, and Vaatu reacted badly and started seeking utter control over the world. While Raava initially did not want to have everything in the responsibility of a sole being, she eventually realized that in order to keep the world as "balanced" as possible, she would have to keep him restrained. After a long time, she deluded herself into thinking that she always had been restraining and fighting him. 

This falls into the same head canon as this: loveormadness.deviantart.com/a…