Its the old Human Nature argument. There is no definition on what this fabled Human Nature is and I find it incredibly boring to reduce the nature of our species to one aspect of us, most people are selfish from time to time. But if selfishness is what define us, most people would be raging assholes and that does not square with my experience of the world.

Further reading: https://www.theguardian.com/commentisfree/2010/nov/20/human-...