Omne agens agit sibi simile. (Each agent produces an effect similar.)
It is through form that an effect bears some similitude to its cause. "Thus the house that is realized in matter proceeds from the house existing ideally in the mind of the architect. In the realm of nature, likewise, man begets man." The form of that which is generated is also the end of generation, and it is through the achievement of this end that things become like the agents that produce them. This is true for both natural and artificial objects. The more something fully becomes what, in some sense, it already is, the more closely it is related to its cause. Even if an effect cannot rise to the perfection of its cause in a specific way as when man procreates to form another man, something of the same species, an effect will still imperfectly achieve some likeness to its cause.
God as the first cause is also the first agent. All creation, then, seeks some similitude to the form of its first cause. This form, though, is nothing else than God's goodness. "This, then, is the reason why all things were made: that they might be assimilated to the divine goodness."