In a number of places, the Bible warns of a hardening of the heart, "as in the provocation," referring to the Children of Israel who were quick to turn away from God even after tremendous demonstrations of His power and goodness: released from Egyptian slavery, miraculously escaping through the Red Sea, food and water provided in the wilderness for a huge mass of people, etc. The hardening of the heart is exemplified in the spoiled child who is always demanding something more as soon as he receives what he wants.
In my own life, I have had some experiences of physical healing and meeting of financial needs, etc., which were dramatic at the time with no other explanation than something bigger than you or me operating! But over time, they lose their luster, unless I make the effort to remember in gratitude.
What might be the Science, the principle behind gratitude? "No good is, but the good God bestows." (S&H) An acknowledgement of good is an acknowledgement of the presence of God. A constant wanting things you do not have is a constant declaration against God, a hardening of the heart. This self-destructive behaviour must be guarded against by the constant cultivation of expressing gratitude.
As Eddie, the man of God referred to in a recent post, said the other day: "I was driving along just thanking God for this and thanking God for that - I wake up in the morning thanking Him, and go to sleep at night thanking Him." When he went to the eye-doctor for a check-up, (he confessed he was worrying: "We spend too much time worrying!") the doctor marvelled at the healthy condition of his eyes. No wonder!