There are many quotes that could be used to illustrate that happiness and fulfillment are products of being generous toward others, since that is one of the major lessons Scrooge learns from his experiences with the three spirits.
Scrooge's nephew sets the theme out when he invites Scrooge to his home for Christmas dinner. He describes Christmas as being
the only time I know of, in the long calendar of the year, when men and women seem by one consent to open their shut-up hearts freely, and to think of people below them as if they really were fellow-passengers to the grave...it has done me good, and will do me good; and I say, God bless it!
When the Spirit of Christmas Past brings Scrooge to the memory of the Christmas party hosted by the Fezziwigs, Scrooge recognizes the joy that they received from sharing their love and life with others.
He has the power to render us happy or unhappy; to make our service light or burdensome; a pleasure or a toil. Say that his power lies in words and looks; in things so slight and insignificant that it is impossible to add and count 'em up; what, then? The happiness he gives is quite as great as if it cost a fortune.
As Scrooge enters his new life of generosity, he begins to understand the wonders and rewards of this new outlook.
I'll send it to Bob Cratchit's," whispered Scrooge, rubbing his hands, and splitting with a laugh. "He shan't know who sends it. It's twice the size of Tiny Tim. Joe Miller never made such a joke as sending it to Bob's will be!"