"And Jesus said to him, 'Why do you call me good? No one is good except God alone.'"
Goodness is a confusing thing. What does it mean to be 'good' and what does it mean to perform 'good' acts? After all what is the underlying quality that renders good things consistently good? On one hand we congratulate international athletes for being world-class and 'good' at their sport, yet we also declare non-athletic children 'good' when they participate in low-level sporting events. Clearly the child and athlete are poles apart but their disparate acts are still 'good'. For the Christian we are encouraged to be good, but really what does that mean? Jesus asks the question in Mark 10:18: why do we call him good?
One preconception of Christians is that we perform good acts in some attempt to achieve a standard of behaviour which will endear us to, or worse hoodwink, a heavenly arbiter into allowing us into heaven. Indeed, this is unfortunately the thought of many professing Christians today. The Bible though makes it clear that good works are not salvation in themselves (Jer. 6:19-21; Hos. 9:3-5) let alone the issue of universal sin (Rom. 3:22-24). But yet we are encouraged to show faith with works (James 2:18-24). All rather confusing. Moreover, on a cosmic scale, who would ever want to worship a God who you could reach by works? Would He actually be just and worth trusting if we could achieve His level of holiness for ourselves? Would you trust any human for your salvation? (This is an appeal to the ontological argument).
The issue still remains: what characterises a good act and why should we perform them? What is it within the act which endows it with the desirable quality of goodness? Mother Theresa, for instance, is often held as a paragon of good works but I would imagine she would never consider her acts good, but rather the right thing to do.
Good acts are usually directed towards others whether they are conscious of that act or not and in some way they become 'enhanced' or reap some benefit from that act. What is Christian reasoning behind this? Well, I think that at their core good acts are those which endow those who bear the image of God (Gen. 1:26) with dignity. Think about it: Mother Theresa's acts dignified the poor, turning the unloved into the loved. She refused to accept them as 'lesser' people but could see they bore the image of God within them: "This is Jesus. . . In distressing disguise." See adopted Matt. 25:31-46 into her heart.
By performing good acts we glorify the image of God in others and as a consequence glorify Him. Consider Jesus' miracles: healing the woman with the haemorrhage and Jairus' daughter (Mark 5:21-43), healing the man possessed by Legion (Mark 5:1-20) and the lepers (e.g. Matt. 8:1-3). These people were effectively the 'living dead' excluded from the nation of Israel and therefore under judgement - yet Jesus reaches out to them and by doing so endows them with dignity and healing to return to 'life'. Note as well He actually touched the 'dead', an act forbidden by Levitical law thereby showing that good acts cannot be restrained by death - they have no enemy. Furthermore, is not the mere presence of God Incarnate an act of dignity bestowed upon all man? Was God not saying "You matter"?This is a rather satisfying turn of events which harks back the the first Westminster confession of faith.
By endowing others with dignity and serving them is to engage in the Trinitarian model of relationship and if that model is appropriate enough for God then how much more us. Your acts of kindness and goodness say to that person: you matter, just as Jesus speaks to "You matter" to us. By engaging in these acts we transcend creation and enter into activities divine and by so doing glorify God. Win-win.