Why couples look alike

It's a fact that people generally end up going out with people who are broadly as attractive as they are. We also go for people who are roughly the same height, build, race even. In the case of height, men tend to go for slightly shorter women, and women for slightly taller men, but not more than a few inches different. There are certain... "practical reasons" behind this. We're only animals at the end of the day. However, interestingly, there's another reason that old married couples tend to look alike, to some extent at least. In our daily lives, we will mimic the gestures, speech style, and facial expressions of those around us. Since we spend so much time with our partner, their expressions will be copied to a greater extent. This means that couples will tend to act in a similar manner, especially in ways that we don't consciously notice so much, leading us to perceive them as similar. However, this isn't the only effect. Over the course of many years, the similarities in facial expressions will lead to an actual change in the muscle structure of the face, and a corresponding change in appearance. Which is why your grandparents look kinda similar to one another.

What Gnomatron said is generally true, but that's how our cultures and societies raise us. We are taught to look for a significant other of the same race, class, and religious background as ourselves, instead of looking at a person's personality. Ideally, couples will look alike because underneath it all, they have the same beliefs. Not necessarily religious beliefs, either, but a similiar outlook on life. Couples are supposed to be two parts of the same person, so it only makes sense that both halves of the pair would appear to look alike on some level, no matter what their race, gender, or physical appearance in general.