The act of smiling is and automatic impulse of our nerve system, therefore we must have good feelings before we could produce a joyful and natural smile.
But if we lack self-confidence or if we are filled with sorrows and doubts, we cannot or can hardly smile.
If we pretend to smile, the reaction against other person deep inside our mind will make us produce a stern smile and a smile that is controlled which we called "force smile" which can immediately be identified as an unsincere one.