We need each other to do things that we can't do for ourselves. If we are intimately connected with each other, we just give things to each other; if we don't know each other we find another way to handle it. If you think about it, each according to his or her abilities and each according to his or her needs is sort of the same thing as supply and demand.
The fascinating thing about standard economic stories is exactly that: they assume that everybody wants that kind of closure. That all human relations are forms of exchange, because if everything is an exchange then it's true that we're both equals. We walk up, I give you something, you give me something, and we walk away. Or I give you something, you don't give me something right now, and you owe me. So if we have any ongoing relationships at all, it's because somebody is in debt.
