And you thought a smile is a smile is a smile! Not at all - your mouth is the first point of contact when interacting with people, and apart from the eyes, the mouth is also where various emotions are first visible (a grin, a smirk, unhappy, and so on). There are two ways regularly used to analyse your smile - the emotional evaluation and the objective analysis.
A smile makes you real. If you're afraid of appearing vulnerable when you smile, accept that any form of vulnerability suggested by a smile is precisely what makes it such a powerful act. You're opening yourself up to others and people will respond better to that than to a serious, unmoved demeanor. Moreover, a smile accompanied by an assertive personality will take care of any sense that people will use your smile to walk all over you!
Although it's one of the most instinctive things we do (babies smile, blind people who have never seen a smile, smile), it's difficult to do on command. Someone wants to take your picture or you're introduced to a stranger or your neighbor sticks a baby picture in your face. You try and slap one on, maybe show some tooth. It looks either fake, lame, or scary.
For the objective analysis, the best thing is for you to stand in front of a mirror. Now, smile at yourself using your 'normal' smile. Next, look at the mirror and think of a hilarious moment in your life and give a big, laughing smile. This big smile is probably a much larger smile than you feel comfortable using much of the time if you are not happy with your teeth. When your smile is improved, however, your big smile appears much more spontaneously because you look (and feel) great! So let's figure out what's holding your big smile back -- what it is that bothers you about your teeth and teeth whitening.