Our Sages teach:
Why does the Torah exact such retribution from ingrates? Because ingratitude is a semblance of the denial of G-d. And one who denies belief in Hashem is, in essence, an ingrate. A person who denies the good that his friend has done for him will tomorrow deny the good that Hashem his Creator has done.
In this teaching, our Sages equate ingratitude with heresy. The most basic meaning of ingratitude is to cover, as in covering a pot. An ingrate seeks to cover, to deny the fact that good has been done for him, to minimize another person's kindness towards him. He rationalizes, "Why should I feel grateful to him? He didn't really do much for me. I owe him nothing in return."
One who is ungrateful towards Hashem denies all of the good that Hashem does for him. His ingratitude implies that all the good that happens in his life is not granted him by Hashem. This is a form of heresy.
Abraham, in a generation of idol worshipers, came to recognize the one true G-d. He did this by contemplating the world, with its infinite blessings, its symphony of creations working together in perfect harmony, and asking himself: "Can it be that this palace has no director?" The Director gazed upon him and said, "I am the master of the palace." (Yalkut Shimoni, Parashas Lech Lecha)
Rabbi Eliyahu Dessler comments, that in truth there are no real heretics. Someone who denies belief in a Creator does so because he refuses to acknowledge Hashem's goodness. If he did acknowledge His goodness he would feel obligated to obey His Word - and the "heretics" of this world are not willing to do that.
For since the creation of the world His invisible attributes are clearly seen, being understood by the things that are made, even His eternal power and Godhead, so that they are without excuse, because, although they knew God, they did not glorify Him as God, nor were thankful, but became futile in their thoughts, and their foolish hearts were darkened. (Romans 1:20-21)
Hakaras hatov, literally 'recognizing the good', commonly rendered 'gratitude', means appreciating everything - the small things as well as the great. Grasping the magnitude of hakaras hatov will transform a person's life.
On the third day, G-d commanded the earth to bring forth trees and vegetation, and so it happened. But later, the Torah states: . . . now all the trees of the field were not yet in the earth and all the herb of the field had not yet sprouted, for Hashem G-d had not sent rain upon the earth and there was no man to work the soil (Genesis 2:5).
The verse says that the reason plant life had not appeared was because there had been no rain and no man. This puzzled the Sages and they wondered what this could mean. Rashi suggests that the trees and the vegetation were indeed created on the third day, but were still below the surface of the earth. They could not come to fruition until there was rain, and that G-d did not let the rain fall until there was a man to appreciate the need for it, to pray for it, and to feel gratitude for it. Hakaras ha tov - recognizing the good and being grateful for it.
Without vegetation, life could not exist - and without a human being to be grateful for it, there could be no rain and no vegetation. G-d created the potential on the third day, but the potential could not come to fruition until there was a human being to pray, to appreciate, and to express his thanks. Only man can understand G-d's goodness and thank Him for His blessings.
I truly believe that hakaras hatov will fortify all our relationships. Nothing is due us. Nobody owes us. If we learn to acknowledge G-d's constant kindnesses, we will love Him and forge a powerful connection to Him. Hakaras hatov ennobles us and enables us to be true servants of Hashem and forms the basis of all our relationships.
Atheists believe that the world 'just happened'. The fundamental belief of an atheist is his lack of a relationship with a Creator. Living a life of gratitude rectifies relationships and makes it impossible for one to believe that blessings 'just happen' - every gift has a giver.
Let him who is convinced that his views are true and right express them . . . at every opportunity . . . without considering how much support or how much opposition he will encounter. Only falsehood is in need of many supporters in order to win the day; falsehood must have the authority of numbers to make up for what it lacks in justification. Truth, by contrast, will always prevail, even if it takes time. Noble, courageous and pure, expressed with all the fiery zeal and conviction and with all clarity of sure awareness, stated again and again at every opportunity, truth will ultimately gain respect and admiration even of those who do not accept it. The only truth that can be lost beyond recall is that truth whose adherents no longer have the courage to speak up candidly on its behalf. Truth has never gone down in defeat as the result of opposition, it has done so only when its friends are too weak to defend it. - R' S.R. Hirsch