I was thinking more of an Aristotelian natural purpose rather than a Darwinian one. Of course the Aristotelian conception is a difficult one because you have the theistic tradition pushing it in the direction of intended purpose, and the Darwinian materialist tradition pushing it in the direction of Darwinian function.
But I think something can have an intended purpose without that purpose resulting in the fulfillment of the creaturely nature. If Calvinism is true, it is the intended purpose of some people to provide God the opportunity to display his wrath against sin and to serve as object lessons to make sure everyone up in heaven knows that they got there by grace, but in fulfilling that divine purpose, their desires are everlastingly frustrated. Or, to take a simpler example, we raise some animals for food. Sometimes we do in a way that pays attention to their interests (kosher laws suggest this way of thinking) and sometimes people simply exploit the animals, as in the case of veal calves cooped up in tiny pens. So I think we need the idea of an inner purpose that involves the fulfillment of the nature of the created object.
I an inclined to think that a concept of love could be developed as an active desire for the fulfillment of another being. X loves Y just in case X wants Y to fulfill itself, and does whatever is possible (consonant with other moral duties X might have) to make it possible that Y fufills itself.