The kicker is that being around people for a long period of time is still quite draining, regardless if the shyness kicks in or not!

Also - it seems like there's a connection between people who have (often unrealistically) high expectations for themselves and self-doubt that manifests as shyness/unsureness around other people. I think those who have, for whatever reason, an internal 'critiquer' voice/perfectionist streak have a greater tendency towards shyness. If one can get that voice to shut up somehow (for instance as a result of a series of successes in social circumstances, forced or otherwise!), maybe shyness recedes. Just a theory.