At the tail-end of his uncompleted Compendium of Theology Aquinas treats the question of why we must pray to God for what we hope. First, he notes that we belong to him as an effect does to a cause. He has made us with a purpose in mind which it is his aim to see fulfilled. If a pot were rational and could hope, it should hope in the potter who shaped him. “Thus we are told in Jeremiah 18:6: ‘As clay is in the hand of the potter, so are you in My hand, O house of Israel.'”
But Aquinas does not simply want us to know that we should pray to God our Maker, but how we should pray to God: with complete and utter confidence.
The confidence which man has in God ought to be most certain. As we just intimated, a cause does not refrain from rightly controlling its product unless it labors under some defect. But no defect or ignorance can occur in God, because “all things are naked and open to His eyes,” as is said in Hebrews 4:13. Nor does He lack power, for “the hand of the Lord is not shortened that it cannot save,” as we read in Isaiah 59:1. Nor is He wanting in good will, for “the Lord is good to those who hope in Him, to the soul that seeks Him,” as we are reminded in Lamentations 3:25. Therefore the hope with which a person trusts in God does not confound him that hopes, as is said in Romans 5:5. (Compendium 2.4)
Why should we have utter confidence in prayer? As it always seems to be with Aquinas, because God, that’s why.
Soli Deo Gloria