Creatures are good because God put into them all the good that they have. We see them diffuse this goodness to other creatures. When creatures imitate God by diffusing good in the world, they are doing what good does and are reflecting the good that their Creator put in them and which He diffuses in the world.
For example, we see the sun – which is good – shining forth good into the world for the good of other creatures.
We see plants – which are good – spreading the good of their lives through production of seeds and promotion of further plant life.
We see animals (including Man) – being good in their nature and spreading this good by fostering offspring.
Although none of the world’s creatures – except Man – can think and reflect, nonetheless they all diffuse the goodness God put in them, by following their natures.
Among the world’s creatures, Man is special because God gave Man the dignity of being conscious and being a voluntary tool for God’s diffusion of goodness throughout the world.
Man’s ability to voluntarily cooperate with God’s Plan, makes this cooperation an act of much greater worth, just as the voluntary declaration “I love you” from a dear friend (who really means those words) is of much more value than the same words from a parrot which has been taught this phrase.
But the necessary consequence of Man’s ability to serve God freely, is his ability to choose to say “no” to God and to refuse to be His instrument in the diffusion of God’s goodness to other creatures. That saying “no” to God can come in many forms, e.g., cooperating in the murder of innocent babies, or frustrating the primary end of marriage.
Even if we were to leave aside the mortal sins of refusing to follow God’s laws on these matters of procreation, what a terrible, shriveled-up stinginess it is for spouses to refuse to be generously diffusive of the good of human life, as God wants them to be, and instead to choose to be “un-God-like” and refuse to do what goodness does, viz., to generously diffuse itself so that “good multiplies goodness” (as St. Thomas says above).
A Catholic Candle “corollary” to this article: After considering the above article about good being self-diffusive, we can see that there is similarly a terrible, shriveled-up stinginess in Catholics who have received the supernatural life from God (which is a much greater good than natural life) and yet they fail to do everything they can to diffuse this even-greater good, through seeking to bring other souls to this same spiritual life.
Catholics can diffuse this great spiritual good through prayer, sacrifices, the apostolate, and the good example of their own holy life.
Our life is short! Let us make great efforts to diffuse good – especially spiritual good – wherever we can!
 “I have said to the Lord, thou art my God, for thou hast no need of my goods.” Psalm 15, v.2.
 Here is how St. Thomas Aquinas, Greatest Doctor of the Catholic Church, teaches this truth:
It is a property of goodness to diffuse itself; thus, good multiplies goodness.
Lectures on St. Matthew’s Gospel, St. Thomas Aquinas, ch. 25, section 2052. (“Proprie bonum est diffusivum sui; unde bonus multiplicavit bonitatem.”)
It belongs to the essence of goodness to communicate itself to others, as is plain from Dionysius (Div. Nom. iv). Hence it belongs to the essence of the highest good to communicate itself in the highest manner to the creature.
Summa, III, Q.1, a.1, respondeo.