I've been in an ongoing discussion with some of my students and a colleague about the difference between self-service and selfishness. The Bible is full of appeals to rewards that an individual would desire, and its authors simply assume that there is no contradiction between doing something for another that is also benefiting oneself in the short or long term.
Post-Kantian ethics introduces an alternative premise that ruins this assumption. Kant basically argued that virtue is the fulfillment of a duty apart from any consideration for what one may gain from its fulfillment. We ought not to obey a law because we will be rewarded. We ought to obey a law simply because it is right to do so--it is the most rational choice.
But even Jesus endured the cross, "for the joy that was set before him," and the eternal covenant between the Father and the Son was an agreement that rewarded the Son (with the Father and the Spirit) with glory upon the conditions of the covenant. If God Himself is righteous in seeking His own good through the lavishing of good upon others, why should we, as God's image-bearers, consider ourselves bound to a "higher" standard? Really, Kant's standard isn't higher, but rather inhuman. God created men with wills and affections that cooperate with his rational mind to engage in appropriate worship of the Triune God. A Christian should not simply obey out of a sense of duty to the rationally correct choice (although that would be better than disobedience and dereliction of duty!). Rather, the Christian is motivated out of a sense of thanksgiving for what God has given to him, and out of a desire to please God. It is also the case that the Christian obeys from a hope that his obedience will be rewarded by the Father.
The idea of working for a reward smacks of "works righteousness" to the ear of many Christians. Since we cannot "earn our salvation," it is foolish to think of works as a kind of reward, so the sentiment goes. But there is a misconception in the nature of works under this view. Works are something that follow upon salvation, not something that precedes it. A salvation that did not include the fruit of good works would be a salvation that does not save. For what else is the Christian saved into but obedience and wholehearted effort to do that which is right before God and men? No one rescues a plant from withering without expecting it to produce fruit. I didn't adopt my two oldest boys in order for them to languish in idleness. Nor does our Heavenly Father deliver us from death that we might remain stagnant or frozen in our affections or will to do. Rather, God has prepared for us works beforehand, that we may walk in them. Paul refers to the Philippians as his crown. There is a real sense in which the good works we do are really only a benefit to ourselves in a secondary way, because a real work of good never benefits a single, isolated individual. Even if one of the works I do is to cultivate my soul through spiritual discipline, it must of necessity please God to be more like Him, and it must of necessity spill out into the relationships God has put me in. Are not more loving relationships a type of reward? Is not the pleasure of our Heavenly Father a reward? Can we say that our progress in sanctification is not a reward? Is any Christian capable of maintaining that such rewards ought not to be pursued, desired, or otherwise worked out in fear and trembling?
It would take a fairly complex argument to do so, I should think. I'm still waiting to hear one from any of my students, in any case.