Thomas indicates in several places the primacy of the good as final cause. This fundamental principle reveals the secondary role of the good as self-diffusive as a necessary context to comprehend his doctrine of the good. Having that in hand, we can understand how this functions in God and the creaturely realm:
Now each thing acts in so far as it is in act, and in acting it diffuses being and goodness to other things. Hence, it is a sign of a being’s perfection that it ‘can produce its like,’ as can be seen from the Philosopher in Meteorologica IV. Now, the nature of the good comes from its being something appetible. This is the end, which also moves the agent to act. That is why it is said that the good is diffusive of itself and of being.
Everything acts to the degree that it is good. Now, God is infinite and therefore acts infinitely, but human beings act through the goodness that arises in their participation in being. Hence, the greater the participation, the greater the goodness one possess in act, the greater the ability to operate, and more intense is the act. In as much as every being is created, it has the necessary single substantial act of being, namely, existing. Although the soul can be considered an act, it cannot be considered “pure act,” unlike God, who is an unqualified pure act, because the soul of a human being is constantly dependent on God’s gift of being. Since every being desires its perfection, it desires actualization. This is why we can make a distinction of act-potency in the human person. Since, human beings desire the good, but yet do not have it; they liken most to their creator through operation and self-communication of good:
Any perfection which a creature has from its essential and accidental principles combined, God has in its entirety by his one simple act of being.
Because finite beings exist in act, they necessarily desire through activity and receptivity the good that they do not yet have and sharing the good that they do have, albeit not perfectly. As we move up the chain of being, we see a higher degree of substance to diffuse, the ability to diffuse, and its inclination to diffuse.
 ST Ia Q.5 A.1: The essence of goodness consists in this, that it is in some way desirable. Hence the Philosopher says (Ethic. i): ‘Goodness is what all desire.’ Now it is clear that a thing is desirable only in so far as it is perfect; for all desire their own perfection;” De Veritate Q.21 A.1: First of all and principally, therefore, a being capable of perfecting another after the manner of an end is called good; but secondarily something is called good which leads to an end.
 SCG Book I C.37
 De Veritate Q.21 A.5
 The doctrine of the good as self-diffusive plays an important role within the context of creation as well. God by creating necessarily communicates himself. This is pertinent to any Imago Dei theology which has striking implications to the doctrine of theosis.