For Paul, grace remained as a crucial theme of his life to which he returned over and over again because Grace is central to all of Christian life and thought. That's why Paul never begins nor ends his letters without a reference to grace. Paul sees grace as the source from which flow all the blessings which you and I have in Christ. For Paul, grace was nothing less than the unsought and unbought saving activity of God which made him a grateful debtor forever.
Wilkins, Michael J., "Gratitude" (1993). Speakers. 84.