The thing with not smiling (and I'm not a natural smiler, I've learned this and applied it, YMMV) is that people see you not smiling and they fill in the blanks. It's like being quiet around very extroverted people, they fill in the blank (your silence) with assumptions that you don't like them, or think you're too good for the conversation, or that something is wrong. When you don't smile in situations that society expects it people assume something is wrong or you are cold or uninterested in them.
Well introverts don't tend to be initiators of conversation, those that lean towards extroversion tend to have to initiate things.
Once again it's not the introvert that has the problem but the rather the extrovert who believes everyone should be like them. Initate conversations etc... That's not to say introverts won't initate things from time to time.
But there is a subtle difference between being an introvert and being shy. Now it is likely there are far more shy introverts than shy extroverts.
But once again there is nothing wrong with being shy. Shy people tend not to open themselves to many people and should you win a shy persons confidence you could have a friend for life.