I agree that logically it is a bit arbitrary. But in the end, I think that's all we have. Out of all the world that I can perceive, I assign humans the greatest value. Further, I assign equal value to each human. I admit that there isn't really any logical basis for that other than my instincts, but there it is.

It seems to me that a whole lot of people would agree with the notion that we are all equal, but a whole lot fewer would agree with what I see as direct consequences of it.