"It took some time before I finally understood that they were faces," he wrote in an e-mail. In Japan, emoticons tend to emphasize the eyes, such as the happy face (^_^) and the sad face (;_;). "After seeing the difference between American and Japanese emoticons, it dawned on me that the faces looked exactly like typical American and Japanese smiles," he said.
The theory is that you can keep your mouth from going into a smile pretty easily, but it's harder to suppress the display of emotion in your eyes. The Japanese, then, tend to suppress their smiles so they look to the eyes for emotional cues. Americans don't suppress their smiles so they look to the mouth. That's why the different emoticons developed.
Read the whole story here.