Probably the worst enemy of enthusiasm is time.
Human beings have a remarkable and sad capacity for getting tired of wonderful things.
Your first day of vacation on the coast the sunset was breathtaking and made you so happy you could sing. But by the end of your stay you hardly noticed it any more. Vacationers get tired of sunsets, millionaires get tired of money, kids gets tired of toys, and Christians get tired of doing good. At first the excitement of teaching that Sunday School class was strong, but now you have grown weary of well-doing. The thrill is gone.