"To be grateful is to recognize the Love of God in everything He has given us -- and He has given us everything.  Every breath we draw is a gift of His love, every moment of existence is a grace, for it brings with it immense graces from Him.  Gratitude therefore takes nothing for granted, is never unresponsive, is constantly awakening to new wonder and to praise of the goodness of God.  For the grateful person knows that God is good, not by hearsay but by experience.  And that is what makes all the difference." (From "Thoughts In Solitude" by Thomas Merton)

God delights in a grateful heart. For the more grateful we are the more we open ourselves to the abundance of God's love.  An ungrateful heart is a selfish heart, always looking and expecting more but short on the necessary response of true gratitude. The truth is that we can never fully respond to the multitude blessings we have received from God. But a lack of response or a casual response is an affront to God's love. "Lord, thank you" should be a constant prayer of our heart.