I am not an American, but, I think the question is a very good one. I do think people are simply more "detached" from their communites. We have become accostomed to doing donations to charities and being "done with it". Now, of coarse I don't say this is true for EVERYONE, but, for society as a whole.
Here in Ottawa, Canada, we are discouraged by the City and government from giving "Directly" to homeless people and pan-handlers. We are told that by giving to the Shelters the money does more good. This is true, but, at the same time, we have just a little less contact.
Plus, I don't know what it is liek down there, but, here in Ottawa, I can't walk down any busy downtown street without being approached at least once by someone wanting change. In fact last Friday while waiting for a bus to go home at 11:15 PM, I was asked, once for change, asked if I would buy a Whopper and Large Fries for $1.00 from another guy, and for "Chnage for the bus" by a third, and offered "Weed" from a fourth... I was only at the stop for 15 Minutes.
It happens so much, that we sometimes stop listening. Only at certain times of the year do we really do more, (Christmas, Thanksgiving, Easter, etc...) and those are the times of the year when we are really pushed to donate.
I agree that The United States probably does give more to charities than most others, but, I wonder if the majority of that help is given "year round" or just at those few holidays, or when some terrible disaster occurs.
I think another thing that contributes to the "hardening of hearts" in the USA, is the media, in Canada, we have the unique perpective of getting both American and Canadian channels on our cable and satelite services. we can watch CNN, Fox, ABC, NBC, CBS, and we can watch CBC Newsworld, CTV Newsnet, or Global.
Having watch both my entire life, I will tell you, that there is a huge difference. American news is much more negative than Canadian, you always here about shootings, robberies, fires, the War in Iraq, but not a lot of good news, like local people helping there neighbours. Here in Canada we get a lot of the good news. Yes, we get a lot of the bad stuff too, but, not as much. Plus, American Media sensationalizes (sp?) everything. No matter how small the shooting, or robbery it is told as if the world nearly ended.
When people see that all the time, they become more guarded and less helfull I think. I also think that is why when you go to smaller towns and Villages, they are so much more friendly and welcoming. This is true weather your in The USA, Canada, The UK, or just about anywhere else, cities are less friendly than small towns.
As our lives continue to speed up, the amount of negativity we are exposed to (weather walking down the street., watching TV, listening to Music, or surfing the Internet) grows. The more it grows the more we'll have to work to not lose our caring nature.