When it comes to human populations, there's an inverse relationship between density and friendliness. That isn't to say that people in big cities are necessarily unfriendly, but that in heavily populated areas, folks are just less likely to acknowledge each other than when there aren't so many people around.
It's evident when driving. In any rural area I've ever been in, if you drive by a local on a remote county road, he's probably going to raise a finger or two from his truck's steering wheel in a kind of wave. When you get into a town of any size, the waving stops.
I see it when I'm out riding my bike. Away from the city, on roads and paths that don't see a lot of two-wheeled traffic, cyclists generally greet one another as they pass. As you get into more heavily-traveled regions you're lucky to get an "on your left."
It's especially obvious on hiking trails. If you get out there early enough, everyone you encounter, and it usually isn't that many, will offer a "good morning!" The farther you get from the trail head, the better the potential for a brief conversation. If you meet someone on a seldom-visited summit, they just might talk your ear off. Comparatively speaking, anyway. But on your way back out, when the afternoon crowds are out for their weekend constitutionals, the pleasantries begin to erode until you're lucky to even get a nod of acknowledgement.
I get that, when there are a lot of people around, it becomes impractical to say hello to every single one of them. And I don't go out hiking or riding my bike so that I can socialize with strangers. Generally, that's my alone time and I'm out there to absorb the scenery and quiet focus not commonly afforded me at home.
But I'm not really out there to escape. I'm driven to the outdoors by joy and love for the trail. I see the same thing in a lot of the faces I meet. But I can also see that not everyone is there for that reason. And that's fine. One of the great things about Nature is that it can mean so many different things to different people at different times.
Maybe part of the reason for this density-to-friendliness ratio is that sparseness gives us more sense of identity within a Tribe - a feeling that there's something that unites and sets us few apart from everyone that isn't out there. But as density increases, it becomes less obvious what common thread might bind us together. Or the commonality becomes less relevant as numbers increase. Whatever the case, I try to hold in my mind the reason that I'm out there and let it remain untouched by whatever indifference may emanate from the afternoon crowd. They may not be the reason I'm there, but I almost always will have a smile, a nod, or a "Hi!" to share with them.