From Scientific American:
We smile because we are happy, and we frown because we are sad. But does the causal arrow point in the other direction, too? A spate of recent studies of botox recipients and others suggests that our emotions are reinforced—perhaps even driven—by their corresponding facial expressions. Charles Darwin first posed the idea that emotional responses influence our feelings in 1872. “The free expression by outward signs of an emotion intensifies it,” he wrote. The esteemed 19th-century psychologist William James went so far as to assert that if a person does not express an emotion, he has not felt it at all. Although few scientists would agree with such a statement today, there is evidence that emotions involve more than just the brain. The face, in particular, appears to play a big role.
This February psychologists at the University of Cardiff in Wales found that people whose ability to frown is compromised by cosmetic botox injections are happier, on average, than people who can frown.