It's not just us. It's nature
Whether you're talking about termite or ant colonies, elephant herds, or troops of baboons or chimps, the need for space and territory are universal traits. The group mentality will not allow the sharing of resources with others.
Preach peace and brotherhood all you want, but the reality of the world today proves that I'm right. If we don't create new space to spread out in, then natural instincts bred into us over a billion years of evolution will force a population correction to maximize the finite space we have now.