I'm intrigued that it should be seen as polite to strike up conversation with someone who has no inclination to do so, and that those who wish to keep to themselves might be regarded as behaving in a rude or uncivilised way - it's not that someone like myself is snooty, just shy, and really only comfortable talking to a small circle of people: approaches from complete strangers then are discomfiting, and to me feel like invasion of my privacy. Aquarelle's description of the social greetings in shops and waiting rooms is a different matter - it's a clearly defined formula (though where I went when on holiday in France, I noticed that the menfolk came first in the greeting, which was a very swift "bonjour, monsieurdame", all squished up

.), and has a stock reply, which requires nothing further. The problem with the "how are you?" is that it appears to need a considered answer, and means that one has been asked to decide how much is appropriate to say, from "fine thank you" through a brief discussion of the weather and the economy, to a detailed history of the last week. For me it's not so much the social effort required as the fear of getting it wrong, and for that reason I'd rather be left to my own devices.
That causes a dilemma, of course

. How on earth are we to tell ahead of time who wants complete strangers to strike up chat, and who wants to be left in peace?