Forgive the pontificating, but I didn't get to teach my classes last week because of the hurricane, so...
The aesthetic preference for bilateral symmetry in inanimate objects is a by-product of the evolved preference for the same in the context of mate selection, bodily symmetry being a marker of developmental stability and of underlying genetic quality.
We don't care about top to bottom centering as much because faces and feet ain't supposed to look alike.
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