Why is it the people are selfish and greedy?

Why is it the people are selfish and greedy? It is a difficult question. I think that most people go through stages of being greedy and self centered, and and there are some people who are just damn right greedy, most of the time. What makes us want to think of only ourselves...? Well maybe it's a preservation technique...? maybe we just try to get the best for ourselves, 'cause it helps us live more easily... In the end, I believe that humans are social beings, and that we need each other, to survive... So it would make sense to not be greedy and selfish, 'cause to do so would mean distancing those people around us who can actually help..So be selfless, everybody...and make more friends...Mondo