Sometimes mimicking another person's behavior doesn't come by choice.
Research has shown that certain actions are the result of social cognition. For instance, when we hear the sound of laughter, our brains automatically tell us to join in — even if we don't get the joke.
Some scientists point to mirror neurons — brain cells that fire when we watch someone else do something. Others think it's tied to personality traits like empathy or neuroticism. But the short answer is we still don't know exactly why it happens.