Why is everyone manic depressive?

I can't handle any of these people whose moods swing like a pendulum traveling from one emotional extreme to the opposite emotional extreme at a drop of a hat. I realize my moods run the gamut but the only difference being that I don't externalize my moods the way that other people do. Tired of walking on egg shells every time I'm around one of said people until I can gauge which end of the spectrum they are on.

I can't deal because I internalize everything. Making myself feel horrible because of someone else's mood. For a while I never understood why I took other's moods so personally, but now I know why. When I care about people, in any small way, I feel powerless when they reach the depths of emotions, I want to be able to fix it, but I can't, I know I can't. I know when I am in the dumps that no one or thing can retrieve me other than my own shear will to see it through to the other side.

But really, it is not so much my need to fix them as it is my need to not feel like I have failed them in some way. I don't know where this constant sense of failure comes from, as if nothing I do will ever be what is needed or even what is wanted. But here is the truth, nothing I do will ever be good enough to satisfy myself and my own expectations.

So I am left wondering what I did like I'm a little girl who blames herself for her father's unhappiness. I can't control other people's happiness, so why do I let it effect my own. Maybe someday I won't let it get to me the way it does. I might learn that I can't be in charge of the emotions of others. It has nothing to do with me, I did nothing wrong. And maybe someday I will actually say that to myself and believe it.