I have the same problem with smiling, and making facial expressions, and been going about it all wrong the last 3 years by trying to learn how to "make facial expressions." Until recently I thought it was just a nonverbal communication disorder
http://en.wikipedia.org/wiki/Dyssemia tho now it seems like more if an effect of my unability to be emotionaly open to people.
We can't just make facial expressions without feeling emotions, because its a reflection of our emotional state. Actors are good at this because they can easily bring themselves to feel emotions and their facial expressions, tonality and body language flow with it when they play the part.
We smile automatically when laughin because we are emotionally open, let our armor down and living in the momet. No wonder its so eazy for people to connect over laughter.
Think about a time you were just feeling really good and smiling, then recreate it in your mind, this is a basic NLP trick. If you can actually be present in your interactions, by staying grounded in the environment, direct social tension to your advantage and at the same time being open and vulnerable to expressing your emotions you can tune in to your internal state and direct your energy. Smiling at someone is giving them an emotion, directing your emotional energy at them.
Prtty much how naturals spark attraction without saying a word. Awaken the natural within all of us.