It would seem that if Smith hates Jones, then Smith has a reason to try to harm Jones. And if Smith on the contrary loves Jones, then he has a reason to work zealously for Jones' sake, since the beloved is "another self," and their wills unite and their hearts indwell in each other through charity. Finally, is Smith is indifferent toward Jones, then he should be willing to use Jones for his own advantage and profit from his existence and actions within society as part of social cooperation, free market, etc.
But in each case, Smith would seem to receive his reward in full. Even in the second case of love, Smith freely rejoices in Jones' happiness. Whence then morality?
The answer is that the causal relation between feelings toward others and deeds toward them goes both ways.
Thus, Smith's hatred for Jones can cause Smith to injure Jones. But injuring Jones likewise deepens Smith's hatred. If Smith on the contrary "forced himself" to do good to Jones, then his hatred for Jones would likely diminish. Enough of such seemingly "unmotivated" good deeds, and Smith might rid himself of hatred for fellow men completely.
Similarly, it's not just that charity toward neighbor causes good deeds; but good deeds tend to increase one's charity, though not necessarily in an obvious fashion.
In this sense, natural morality involves the intellect purifying the will of violent hatred. It suppresses, often through prodigious effort and painful self-abnegation and penance, existing vicious desires, substituting at first "indifference," non-aggression, abstinence from vengeance, and willingness to cooperate within society for mutual benefit. Therefore, morality motivates not through a desire but through a purely intellectual grasp of one's duty which commands that certain desires be purged from the soul.
Christian morality goes further and enjoins not just peaceful self-interested cooperation but active love for fellow men. It therefore demands that one grow in charity throughout his whole life. Again, a Christian may not feel any particular charity at first, but through holy works, this charity is sure to increase; that's how the world and human beings work. The motivation is an understanding of one's grace-enhanced moral duty, not any desire. If the motivation for a good deed is 100% existing charity, then moral progress might not occur. St. Thomas even divides people into beginning, proficient, and perfect in virtue:
For at first it is incumbent on man to occupy himself chiefly with avoiding sin and resisting his concupiscences, which move him in opposition to charity: this concerns beginners, in whom charity has to be fed or fostered lest it be destroyed;
in the second place man's chief pursuit is to aim at progress in good, and this is the pursuit of the proficient, whose chief aim is to strengthen their charity by adding to it;
while man's third pursuit is to aim chiefly at union with and enjoyment of God: this belongs to the perfect who "desire to be dissolved and to be with Christ." (ST, II-II, 24, 9)
For some reason, this elementary dynamics, long well-understood in the Christian world, seems to escape modern moral philosophers. For example, Lester uses the strange and objectionable term "altruism" and, though defining it reasonably as "an interest in other people... as ends in themselves," fails to connect it to love of friendship.