It's a genetic problem. Women are hardwired to smile more in front of people, even if they're unhappy (1). They're more polite, right from the moment they begin to speak (2). They try much harder than men to avoid conflict (3).
1 Cole - Children's spontaneous control of facial expressions - Child Development (1986), Saarni - An observational study of children's attempts to monitor their expressive behavior - Child Development (1984)
2 Politeness: Some universals in language usage (1987), Leaper and Ayres: A meta-analytic review of gender variations in adults' language use, Personality and Psychology Review (2007)
3 Campbell - Staying Alive: Evolution, culture, and women's intrasexual aggression. Behavioral and Brain Sciences (1999)
1 Cole - Children's spontaneous control of facial expressions - Child Development (1986), Saarni - An observational study of children's attempts to monitor their expressive behavior - Child Development (1984)
2 Politeness: Some universals in language usage (1987), Leaper and Ayres: A meta-analytic review of gender variations in adults' language use, Personality and Psychology Review (2007)
3 Campbell - Staying Alive: Evolution, culture, and women's intrasexual aggression. Behavioral and Brain Sciences (1999)