Brain Golfarb points to an interesting article in the Seattle Times (pertinent to me - I've just moved from London to Seattle) analysing the 'Seattle Freeze', a phenomenon Sociology professor Jodi O'Brien describes as 'the phenomenon of the plastic smile.'
Raised in Salt Lake City and Zurich, O'Brien came to Seattle 20 years ago for graduate school and is now chairwoman of the sociology department at Seattle University.
"At the university, where people are hired from all over, this is a pretty standard conversation," O'Brien says. "Seattleites are often seen as having this veneer of pleasantness but being hard to come to know."
Something similar to the phenomenon described goes on in London too. Speaking as a Londoner (for most of my last 28 years) I know plenty of people who live in London for 2 years plus and yet never properly get to know any Londoners at all - they get to know plenty of fellow 'in-transits', but no perma-residents. One theory (mine at least) to explain this is to understand the transcendental nature of a large proportion of London's population. Most Londoners weren't born there, most will move out, and plenty are there for less than 5 years....so in fact there really aren't many 'Londoners' to get to know, and those that Londoners meet are generally nomads - in town for a while but off somewhere else soon.
Now, maybe it's because I'm a Londoner (I feel a song coming on...), but I think it is human nature to tend to feel that investments you make in terms of friendships (i.e. time, emotional support, sacrifices, compromises, etc) will tend to be biased toward the more permanent members of your community, and less to those that you know (or suspect) will skip away just as you get to know, leaving you high-and-dry.
Anyhow, interesting sociological article.