Selfishness, if taken to an extreme, eliminates human camaraderie. It too often leads to the pitfalls of prisoner's dilemma. Existing purely for our own sakes erodes the invaluable social capital we've built to make for a more pleasant life. In order to maintain and propagate this social capital, we must be fully willing to contribute without intentional gain to the world in which we live. In building up our communities, we are building up our own potential for a happier existence.
Life is an ongoing negotiation. We must be constantly open to expanding the pie instead of dividing it in order to best satiate both our fellow negotiators and ourselves.