The most basic instinct for a human is to survive, and secondly for the human's offspring to survive. Short term benefits will always seem more attractive than long term payoffs in this struggle to survive. Survival, historically and evolutionarily, means looking after your own interests. It makes sense to be selfish. Look after your own, screw everyone else. In fact, our basic psychology allows us to very easily categorise our fellow humans into "us" and "them". "Us" = good, like me, worthy of survival, right, worthy of protection. This category can include just our own family, or our family and friends or our entire country. It very very rarely includes the whole world - there is always someone we will see as threatening our survival, as "them." "Them" = different, dangerous, unworthy of survival, need to be stopped (think terrorists, rival countries, people who hold greater resources than us). This basic, catch-all mentality, that suits individual survival, but not overall world peace, leads to "man's inhumanity to man." It sounds terribly defeatist, but I believe it's the way we are, and it'll take a very long time to change, if it ever does.