Shyness, especially at the adult conversation level, often develops through an odd psychological quirk where you think your efforts are abject failures if they go completely unreciprocated. For example, if you smile and nod at someone, and they do not respond in kind, a shy person is much more likely to castigate themselves for their own failure, whereas a more extroverted person is likely to (correctly) place that blame on the other person for not wishing to interact.