I don't so much think it's that our state is naturally negative, but that those prone to Bipolar Disorder (and depression and anxiety based disorders) are very sensitive to the energy around them. I think our natural state is that we are empathic, and because of that, we pick up on so much around us that others don't.

Let's face it, the world is not a very positive place, in general. Americans are consumed with materialism and are extremely overstimulated and for the most part, lacking in spiritual depth. It doesn't exactly provide someone who is quite sensitive with the healthiest environment to thrive.

My theory (which is just my own nonsense really...lol) is that if we were surrounded by a more loving and spiritual culture that moved at a slower pace, we would be much less prone to the symptoms of mental illness.