Not only is the good of others part of an agent's own good, and so intrinsically valuable from the agent's point of view; promoting others' good is extrinsically valuable. Pursuit and realization of many worthwhile personal projects require diachronic social cooperation, which sometimes requires, and is almost always enhanced and facilitated by, fair terms of cooperation. Fair terms of cooperation require that cooperation be conducted on terms of mutual advantage to all participants. In order to pursue her own projects, therefore, an agent is well advised to make sure that others benefit from cooperative endeavors.
Because an agent's welfare depends upon the welfare of others in these ways, an agent has reason to promote the welfare of others. It will be in an agent's interest to develop strong, relatively coarse-grained social dispositions from which she rarely deviates.
(David O. Brink, Moral Realism and the Foundations of Ethics, Cambridge Studies in Philosophy, ed. Sydney Shoemaker [Cambridge: Cambridge University Press, 1989], 243)
