If we are not grateful to God, we cannot taste the joy of finding Him in His creation. To be ungrateful is to admit that we do not know Him and that we love His creatures not for His sake but for our own. Unless we are grateful for our own existence, we do not know who we are, and we have not yet discovered what it really means to be and to live. No matter how high an estimate we may have of our own goodness, that estimate is too low unless we realize that all we have comes to us from God.
The only value of our life is that it is a gift of God.
Gratitude shows reverence to God in the way it makes use of His gifts.