Smile has power to change negative to positive in our own experience, we can counteract the effects of pessimistic thinking by forcing a smile for example. Even a forced smile makes our brains and bodies to re-evaluate the situation: “is this really so bad?”
Eventually smile can rewire our experience, and the fake smile can become a real one.
Fake it till you make it, William James said and so does positive psychology.
Smile manifests positive emotions, which in turn create cycle of positive emotions. Emotions are contagious so why not spread some smile?
With smile action can be more fun, pleasant and thus also productive.