I think they do as women are more naturally emotional, expressive and empathetic than men. What is more, they don't have a taboo about showing their emotions as compared to men. Another reason which explains why women smile more is because it's typical for them to have a wish to share their happiness with others or just show their love and care to people around, which helps them connect with absolutely everyone. Well, and I just can't help mentioning one more reason that has just come to my mind - women smile more just because they are expected to smile no matter where they line up on the social, cultural or economic ladder.