What was the main lesson that Scrooge learned in A Christmas Carol?

There are a number of lessons that Ebenezer Scrooge learns in this classic story by Charles Dickens. Perhaps the most important and most far-reaching one is that it is never too late to find happiness. Scrooge is shown, via the ghosts who visit him, that throughout his entire life he has chosen his own misery, and has often caused others to be miserable. He feels guilt at being shown his own cruelty and dispassion. He learns from the visions shown to him that he can choose his own happiness. He sees that Bob Cratchit does his best to be happy with his family, despite being overworked and having very little money. This act of will from a man with limited means shows Scrooge that happiness is a matter or perspective, and that every action may be inspired by the desire to do good. Everything changes when he is given this last chance to shift his own perspective; he becomes generous, compassionate, energetic, and sees the world with fresh eyes.