Contrary to popular belief, one does not participate in an exchange because one is self-interested; rather, one is allowed to become self-interested by participating in the exchange.
Self-interest is not a natural state in which all individuals find themselves; rather, it is a state of pleasure the individual can produce only in becoming estranged from another. I cannot be self-interested – I cannot become engaged in my own experience of grief and reward – unless the other becomes a stranger to me.