I mean, what do you look at when you look a person in the face? Do you watch the mouth or the eyes? This is culturally determined: Westerners usually look at the mouth, because we use the lower part of our face to express our moods and feelings.In Japan (and other countries in East-Asia) it is not common to show your feelings and therefore the mouth is less expressive. That is why we sometimes say that the Japanese are unfathomable - meaning that we are not able to read their code. But we look in the wrong way. Japanese themselves look at the eyes of other people. The eyes are less easy to control and therefore give a more honest impression of your feelings. Not what you want other to see, but what you show unconsciously. Remember this during business conversations: your friendly smile is no use when your eyes are not smiling, too!
Interestingly, this culturally determined way of looking at faces also has influenced the "small faces" of Japanese smileys. Those Japanese emoticons are a world apart. You will find them in much larger numbers than in English and they are very original, as for example the smileys based on manga. What they all have in common is the emphasis on the eyes. So not :-) when you smile but (^_^). The hooks indicate the face here, not the mouth, which is just a hyphen. Because women like to have small mouths, they write (^.^). Here are a few more: blushing (*^_^*), sleeping (-.-), crying (;_;), winking (^_-) and scratching your head while breaking out in a cold sweat <^_^;