[grzm]

Or just the fact that we're all humans and humans tend to have a lot of similarities in their psychological makeup. Any purely logical framework (such as game theory), that doesn't take into account the non-rational aspects of human psychology is going to be ultimately limited in its explanatory power. The reality of human psychology is just as real and objective as human physiology, and the associated variability due to both nature and nurture.

[api]

That just bumps the question back to evolutionary dynamics, largely a subfield of game theory. Why are we "wired" to produce these cooperative "nice" social systems?