When Hashem created the world, He would not send the rains until there was a person on this earth who would appreciate this gift and and be grateful for it. Adam prayed for rain. It fell. Trees and vegetation grew. And Adam thanked Hashem. It was necessary, as soon as man was created, to teach him the lesson of gratitude — because gratitude makes all the difference between a life that is happy and fulfilling, and one that is sad and bitter.