Because we now live in a dominance hierarchy based social system that pits everyone against everyone else - and we suffer the isolation and lack of community that entails -although we are still a cooperative species at heart.

As it turns out it is more than a metaphor — social pain is real pain. With respect to understanding human nature, I think this finding is pretty significant. The things that cause us to feel pain are things that are evolutionary recognized as threats to our survival and the existence of social pain is a sign that evolution has treated social connection like a necessity, not a luxury. It also alters our motivational landscape. We tend to assume that people’s behavior is narrowly self-interested, focused on getting more material benefits for themselves and avoiding physical threats and the exertion of effort. But because of how social pain and pleasure are wired into our operating system, these are motivational ends in and of themselves. We don’t focus on being connected solely in order to extract money and other resources from people — being connected needs no ulterior motive. Scientific American

To be sure, our ancestors didn't necessarily do it because they were noble savages, but the fact that humans are innately a highly social and cooperative species was what allowed us to survive when other hominid strains did not. I'd like to find a way to capitalize on that part of "human nature" again.