I'm all for politeness; don't get me wrong. And my experience of US residents is that they are verbally much more overtly polite than residents of other places I've been.

Pip Radish's post rang true for me, where the antiphon-response nature of thanking was described. But so many of the exchanges we routinely use seem to become automatic and without any intrinsic feeling. When I receive a service (someone holding a door open for me or serving me in a shop) I always say "Thank you" and mean it. In Oz my offering would be responded to, usually, with "No worries." If I were the one providing the service and receiving the "Thank you" my responses would vary between "No worries", "No River Murrays", "My pleasure" and "It's my pleasure" depending on the perceived formality of the situation and my attitude towards that formality

US shop assistants and their Oz counterparts who'd received US-style training routinely would respond to my "Thank you" with "You're welcome" and I've come to regard this as yet another example of US cultural hegemony, so I'll occasionally stir the Oz ones with a smiling reference to their having been really well trained in the US; it's an excellent conversational ambit.

But then, I've also stirred passers-by, amiably, out of their automatic "How are you going?" greeting with my frequent response of "Gently."

Cheers, Rowan