Why mirrors reverse left and right? Eric Lippert provides the best explanation I have seen so far.
“But why then do we all think of mirrors as reversing left and right, if in fact they reverse front-back? Psychologically, humans see a front-back-reversed human as a left-right reversed human. That image of a south-facing person smiling back at you is not an image of a human being at all. Their DNA spirals the wrong way. All their body fat is made out of indigestible Olestra. Their heart is on the wrong side of the body. If we could somehow create a real being who produced exactly that image, down to the front-back-reversed internal structure, there’s no way that they could produce viable offspring with a non-reversed human.
But none of these is apparent at a glance. Since humans have almost perfect left-right symmetry it is extremely easy to interpret an image of a back-front-reversed human as a left-right reversed human. After all, if you were trying to act to look like the person in the mirror, that’s what you’d do – simply reverse your left and right behaviour. You decide what is “slippable” and what isn’t. If you are trying to look like the image in the mirror looks, you’d ignore all that stuff about your DNA and internal organs and reverse left and right, because that happens to produce the image that looks most similar. If humans had different symmetries then mirrors would not appear to reverse left-right at all, because our psychology would be different.”
I never thought about the internal asymmetries in us. :)