Hrm... But "self-interest" is itself a fairly broad category, including many sub categories like emotional state, survival, fulfillment of curiosity, self determination, etc... Seems like it wouldn't be that hard a step, given the evolutionary pressures there have been toward cooperation and such, for it to be implemented via actually caring about the other person's well being, instead of it secretly being just a concern for your own. It'd perhaps be simpler to implement that way. It might be partly implemented by the same emotional reinforcement system, but that's not the same thing as saying that the only think you care about is your own reinforcement system.