I think this is generally true. But I also believe it depends upon where you live.
For instance, in London, you virtually dare not make eye contact with strangers. However, if you find yourself out for a walk somewhere in Yorkshire, perfect strangers will say "Good morning" to you and sometimes strike up an entire conversation.
I have experience of both of the above. The latter weirded me out the first few times it happened, but after a while you get used to it and even enjoy the interaction.
I have my own idea as to why this may be, at least here in the states. As a person that lived in NYC until I was 13 and moved to the Pocono Mountains only to move back to the city when I was 22 (my parents stayed and I of course went back frequently to see them and freinds) I would like to think I have a bit of a handle on both environments.
In densely populated urban areas one is not just in need of a certain private zone around oneself physically that in good taste others should not violate, but also a mental one. Despite it's Liberal reputation, most New Yorkers actually operate in public with a sort of quasi-Libertarianism. If you don't bother me or violate either my physical or mental zone of solitude it's live and let live. I don't care what you do. If I am standing on the corner and there is a woman dressed in Hentai styled Cat costume and on the other side of me is a Cowboy dressed in nothing but a diaper, well as long as they aint bothering me personally I really don't give two airborne copulations, and I am betting they feel the same way. This is necessary in a setting where you rub shoulders all day long with others no matter where you go. You are sharing space so much that corporeally and spiritually you have to give people their space and you hope they do the same. This is not ever a recent phenomenon I think. Compared to the wide open spaces in other parts of the country and the more sparsely populated small towns, places like NYC, Chicago, Phillie, Boston ect. have been big shoulder to shoulder cities for a long time.
Compare that to the rest of the country, especially during the initial drive to spread further west. If you went out to the Indian Territories back in the day, out to the frontier, well, you may not see anyone outside of your family or close by neighbors for months, maybe even years in some places before the 20th Century. So of course it's the reverse of familiarity breeds contempt. If you haven't seen a new face in a while, well you are going to just be the nicest person you can be aren't you? We are social creatures after all. Just as much as in a crowded area you need peace of mind a some me time, so too in desolate environments do you crave new and different social interactions.
Let me also just say that as a New Yorker I get not wanting someone to start talking to me and starting up a covo just out of the blue without a certain kind of personal diplomacy first. Plus, I consider myself a congenial person but City life means being on your toes and aware of your surroundings. Criminals use conversation as distraction sometimes. Finally in urban areas I have found that the types of people who do just chat you up out of the blue are the borderline mentally unstable looking for attention or the full on totally nuts. Sorry if that offends anyone but I've been riding the NYC subway system for many years and that's how I see it. I'm willing to hear counter theories from others though.