Why do good?

I heard the same message 3x now which might amount to more than coincidence.  Why do "good" things?

I was reading some Old Testament and hit this in Zechariah 7:4-10 (bold added):
⁴ Then the word of the Lord Almighty came to me: ⁵ “Ask all the people of the land and the priests, ‘When you fasted and mourned in the fifth and seventh months for the past seventy years, was it really for me that you fasted? ⁶ And when you were eating and drinking, were you not just feasting for yourselves? ⁷ Are these not the words the Lord proclaimed through the earlier prophets when Jerusalem and its surrounding towns were at rest and prosperous, and the Negev and the western foothills were settled?’”

⁸ And the word of the Lord came again to Zechariah: ⁹ “This is what the Lord Almighty said: ‘Administer true justice; show mercy and compassion to one another. ¹⁰ Do not oppress the widow or the fatherless, the foreigner or the poor. Do not plot evil against each other.’


Two days prior Troy Polamalu posted this quote on facebook:
"There are many human activities that are good by nature, but cease to be good when done for ulterior motives. Works such as fasting, vigil, prayer, psalmody, almsgiving and hospitality are good by nature; but not good when done out of vainglory." - St. Maximos the Confessor


And then sometime prior to that (but I was re-hit recently) GK Chesterton struck me with this:
Many people have wondered why it is that children's stories are so full of moralizing.  The reason is perfectly simple:  it is that children like moralizing more than anything else, and eat it up as if it were so much jam.  The reason why we, who are grown up, dislike moralizing is equally clear: it is that we have discovered how much perversion and hypocrisy can be mixed with it; we have grown to dislike morality not because morality is moral, but because morality is so often immoral.  But the child has never seen the virtues twisted into vices; the child does not know that men are not only bad from good motives, but also often good from bad motives.  The child does not know that whereas the Jesuit may do evil that good may come, the man of the world often does good that evil may come.  Therefore, the child has a hearty, healthy, unspoiled, and insatiable appetite for mere morality; for the mere difference between a good little girl and a bad little girl.  And it can be proved by innumerable examples that when we are quite young we do like the moralizing story.  Grown-up people like the "Comic Sandford and Merton," but children like the real "Sandford and Merton."  -'Daily News'

Sandford & Merton was a very popular children's book that has Tommy Merton growing from a spoiled six year old into a virtuous man.  (thank you wikipedia).  GKC's last point was regarding adults looking upon the silliness of the children's book, while children enjoyed it straight forward.