Mood transfer via facial expressions and body language is so powerful that people doing it on a daily basis literally start to look alike. This has been tested with portraits of longtime couples: One set of pictures was taken on their wedding day and another set twenty-five years later. Presented with separate portraits of these men and women, human subjects were asked to match them on similarity. For the set taken at an older age, they had no trouble deciding who was married to whom. But for the pictures taken at a younger age, subjects flunked the task. Married couples resemble each other, therefore, not because they pick partners who look like them, but because their features converge over the years. The similarity was strongest for couples who reported the greatest happiness. Daily sharing of emotions apparently leads one partner to "internalize" the other, and vice versa, to the point that anyone can see how much they belong together.
While I certainly don't want my wife to begin "converging" toward my own ugly mug, I do think there's something beautiful and profound about this scientific evidence. It shows that the connections between us reach every level, down into our very flesh.
Despite being profoundly (and thankfully) emotional and mental, love is also, if you'll pardon the phrase, skin deep.