When it comes to happiness, smile is one of good keys to show that we are sincere, or simply it enable us to express themselves. And smile can be learned, really. Actors, therapists, politician, speakers, and many people have learned to smile sincerely, believably, and deeply.
First, don’t smile at things that don't make you happy or things you take as funny. This will save a lot of trouble. Then, develop a casual smile and learn well that you can pull it even when you are sad or bored. Even, by smiling you will be able change your mood into happiness. Remember, when you smile naturally, your whole face moves. False smiles involve just the mouth, and people notice something wrong. A false grin or bared teeth won't make it. The next time you are smiling, mind of the muscles in your cheeks, forehead, and temples. If you want to be able to really smile with all your face, you will have to practice this repeatedly.
Now about eyes. The real smile can't be faked, it can be judged via the eyes. When something really makes us happy, our eyes get brighter, taking on an excited shine or gleam. So you need to train your mind, just as you trained your face.
Finally, you need to learn to convince yourself that what you are smiling about is having a quality that you like. It means you try to take positive side of those things. If you can't do this, just practice quickly bringing to mind a memory or image that makes you glad. When you have to give your best smile (even if you are sad), quickly think of this glad thought and make that best smile. Then, you are smiling with your whole self.