"Expect nothing in return"
Given we are creatures, we have the inclination to love and worship self. St. Maximus the Confessor tells us 'self love is the mother of all evils.' We are inclined to see what is in it for us. The common slogan sometimes heard is typical of this: He who dies with the most toys wins.
Why would Our Lord tell us that " ...if you do good to those who do good to you, what credit is that to you? One reason ' ... because knowing human nature, He knew that people chalk up favors in their heads. Psychologists speak about reciprocal expectations. If I do this for I can expect you to do this for me. This may be irrational. The other person may not know of these "unilateral contracts" or expectations. But this does not stop people from thinking this way unless they come to see this a problem and work at changing their cognitions.
At the extreme, individuals with borderline or narcissistic personality disorders interpreted and manipulate all that is going on around them so people and events are controlled and/or centered on the self. Even in every day lives, individuals tend to personalize events that are neutral in themselves, but relate these events to themselves (Burns, 1981). This is the normal way of the sinful world. That is why Our Lord tells us in gospel read on the Second Sunday of St. Luke (Luke 6:31-36) in the Orthodox Church: "even sinners do this."
People get their rewards from others. St. John of the Ladder writes "He who loves the earthly rewards (like) fine clothes, oils...perfumes and such...is a true idolater; he seems to honor God, but in reality he wishes to please men, not God."
Let us contrast this to the Divine Emptying of God (kenosis) on our behalf. What does God need from us? What can we add to Him? He is infinite, He needs nothing. Yet He emptied himself for our salvation. His sacrifice is an event of extreme humility. We can never assuage God. We can never make up to Him. Yet He did all for us, with no need on His part. God is pure good who acts out of pure good for good.
Jesus also knew that every sin committed by us is in some way a renewal of the sin of Adam which at its core was pride. We do not see this. We tend to worship self and try to enhance ourselves by the adulation of those around us. To want to eat from the tree of knowledge was for our first parents to try and make themselves God — to have the power of God — instead of knowing their place and waiting patiently for God to tell them what He wanted them to know when He wanted them to know it.
We follow in their footsteps. This gospel is a calls us to act like God — not by assuming God's power, not by letting others reward us, but to act selflessly out of love, expecting nothing in return. What happens when we do this? We become sons of the Most High — adopted sons and daughters so to speak. When we do this we are now acting in the manner God acts out of pure Love.
Don't we get captivated when see or hear of something someone has done, expecting nothing in return? Don't we recognize the "spark" of the Divine when we see people sacrifice themselves for others, even giving their own lives? Don't we marvel when we hear of someone doing good for others even when they have been wronged?
Years ago, in New York City a father whose son had been brutally murdered was asked what punishment the captured killers should receive. The father said "I will pray for their conversion so they could make peace with God." When asked about the death penalty he commented, "It would be a travesty to add death to death. Justice is God's alone." What a beautiful example of Our Lord's love, "Father forgive them for they know not what they do."
Conversely don't we sense wrongdoing or even outrage when people do something for a payoff? Isn't the political cynicism many people share today caused by a sense that many leaders do self-serving things? To have purity of heart and motivation is to have the indwelling of the goodness of God in the center of ourselves. Then we too can be ".. kind to the ungrateful and the selfish" and show mercy to all even as our Heavenly Father is merciful."
Burns, D. (1980). Feeling good: The New Mood Therapy. NY: The New American Library.