Enthusiasm is a learnable skill. And the best people to learn it from are those who are already enthusiastic. Even if someone else is enthusiastic about a specific matter that you have no interest in, you can still pick up from that person how to be more enthusiastic in ways that are suitable for you.
When you were young, you learned from those older than you how to walk, talk, and do many actions. This process came about because we all listened and observed. Utilizing this ability, we can become more enthusiastic by modeling people who are enthusiastic.
Whenever you encounter someone who is enthusiastic, look and listen. See the expression on their face. See how animated they are. Note specifically the nuances of what they do. Listen carefully to their tone of voice. Hear the expressions they use.
Then try it out. As an experiment imagine for a little while that you are this enthusiastic person. What attitudes enable you to be enthusiastic? What is your thinking patterns? In what ways are you speaking and acting differently than usual?
(From "Enthusiasm: formulas, stories, and insights" (2002) pp.144-5; available from http://www.artscroll.com)