It's that last word that I find a little challenging..."anymore." What is it that makes you think that there weren't every bit as many 'cold and heartless' people around in the past - in proportion to the world populace. It's anonymity that has a lot to do with it.
What has always been apparent is that the larger and denser the people are placed, for example, the bigger the city, the less people are caring towards one another. For example, in Australia, in a large city you can walk past hundreds of people on the street and not one of them will give you 'eye contact.' In the outback, where few people live, they'll wave and smile even when passing you in a car.
You could equate this coldness with the hostility which builds when overcrowding occurs. Even gentle white mice in a laboratory become hostile towards one another as they're crowded in and have less and less personal space.
So is it a 'hardness of heart' or a defense mechanism?
I suspect a lot of it would stem from the latter.