Different from other primates, man's face is not covered with hairs. Taking this advantage, man's smile can be easily recognized by other people. After been born, babies cry loudly automatically, but smile could not be found on their faces. Two months later, baby begins smiling.
At that time, babies leave the uterus and are thrown into a totally strange world. They are anxious for parent's love. However, they are too weak to do anything except lying on the infanette.
Therefore, they develop two powerful weapons: crying and smiling. Most of the time, they use crying to express their pain. But, smiling do not always indicates their happiness. They use smiling to hold their parents around them, just like hugging their parents with arms. Therefore, similar with crying, smiling is a method to attract more attention from other people.