"The agent tends to make the patient, not only in regard to its act of being, but also in regard to causality. . . . So, the effect does tend to be like the agent, not only in its species, but also in this characteristic of being the cause of others." 
Is it enough for God to create and sustain us? A deeper look into our relationship with God strongly suggests that many of the things we do are an analogy or an imperfect reflection of the divine likeness. Given that we have been created in the image of God, there is an inclination within each of us to become more and more like God. One of the ways we do tend toward the divine likeness is in our causing other things to exist.
Which is more splendid: a man who builds a house, or the God who made the man who builds the house? It is only on our knowledge of God, implicit or explicit, that we desire to cause things, which sublimely shows how much we want to be like our Creator.
 St. Thomas Aquinas, Summa Contra Gentiles, Book Three: Providence, Part 1, translated by Vernon J. Bourke, University of Notre Dame Press edition 1975, ch. 21.