We do not always act the way economists think we should, mainly because we're both less selfish and less rational than economists think we are. Economists are being indoctrinated into a cardboard version of human nature, which they hold true to such a degree that their own behavior has begun to resemble it.
Don’t believe anyone who says that since nature is based on a struggle for life, we need to live like this as well. Many animals survive not by eliminating each other or by keeping everything for themselves, but by cooperating and sharing.
If you look at human society, it is very easy, of course, to compare our warfare and territoriality with the chimpanzee. But that's only one side of what we do. We also trade, we intermarry, we allow each other to travel through our territory. There's an enormous amount of cooperation.