The concept of goodness comes from our desire for creatures. We seek good food, good clothes, good books, good cars, good televisions, etc. which indicates a degree of goodness or perfection in creatures. All creatures are good to a given degree. If we compare the moon with the cardinal we see that the cardinal in itself is a greater good because is a sentient being with the ability to move, see, hear, smell, eat and reproduce; it has a greater perfection of being.
What do we mean when we say that God is good?
St. Thomas Aquinas reveals the meaning of God's goodness with philosophical clarity as he says: "God alone is good essentially. For everything is called good according to its perfection." (Summa, Pt. I, Q. 6, Art. 3) God is good to an infinite degree, a degree without beginning or end. So only God is good in the ultimate sense of the word. "The will of God tends to things other than Himself inasmuch as, by willing and loving His own being and goodness, He wishes it to be diffused as far as is possible by communication of His likeness. This then is what God wills in beings other than Himself, that there be in them the likeness of His goodness. Therefore God wills the good of everything, and hates nothing. Summa Contra Gentiles, Of God and His Creatures, 96. That God Hates Nothing, No. 2) We as human being can only choose the good. What we call evil is our choice of a lesser good, a good not in conformity with the good of others and our final destiny in God. For example, If I kill a human being it is because it is seen as good for me to be deprived of his or her presence in this world, although it will not be in conformity with God's will.
Further, we experience
suffering in this world as the judgment of God upon our sinfulness. This is
also an expression of God's goodness because he would not be good if he did
not inflict suffering upon our sinful nature in order to rectify its disorders.
"Good and upright is the LORD; therefore he instructs sinners in the
way." (Ps 25:8)
"Thus inasmuch as He wills the good of justice, or of the order of the universe, which cannot be without the punishment or perishing of some, He is said to hate those beings whose punishment or perishing He wills, ... Thou hatest all who work Iniquity, thou wilt destroy all who utter falsehood: the man of blood and deceit the Lord shall abominate (Ps. v, 7). (Summa Contra Gentiles, 96, No. 4)