A thank you should be an acknowledgement of a person's part in God's blessings toward us. Living gratefully ought to be a characteristic of every Christian's life. Tragically, in our fallen nature, self replaces God as the center of what concerns us most. Fallen souls pervert thankfulness into its opposite. The absence of gratitude is an attitude of presumption or entitlement.
People often act as if they deserve all they get, or good is taken for granted.
Warren Wiersbe once told about a ministerial student who was part of a life-saving team. In 1860, a ship went aground on the shore of Lake Michigan near Evanston, Illinois. Edward Spencer waded again and again into the ice cold waters to rescue 17 passengers. In the process, he damaged his own health permanently. Years later at his funeral, it was noted that not one of the people he rescued ever thanked him.