Thomas Aquinas, Compendium of Theology

Diversity among things was rightly established according to a definite order, so that some things might be more excellent than others. For this pertains to the lavishness of the divine goodness, that God should communicate a likeness of His goodness to created things, so far as possible. God is not only good in Himself, but exceeds other beings in goodness, and guides them toward goodness. Consequently, that the likeness which created beings bear to God might be heightened, it was necessary for some things to be made better than others, and for some to act upon others, thus leading them toward perfection.

The basic diversity among things consists chiefly in diversity of forms. Formal diversity is achieved by way of contrariety; for genus is divided into various species by contrary differences. But order is necessarily found in contrariety, for among contraries one is always better than the other. Therefore diversity among things had to be established by God according to a definite order, in such a way that some beings might be more excellent than others.