"Force yourself to act enthusiastic, and you'll become enthusiastic." Frank Bettger.
Think about the people you enjoy in your life. I mean people you work with, or do business with. How about the people at school, church, or the store? Which ones do you like the most? Are they enthusiastic?
Don't go crazy with it. "Enthusiastic" doesn't mean that you are so hyped up that no one can stand you. It means that you really show interest in what you are doing, especially in the people around you.
Read chapter one of Frank Bettger's book, "How I raised myself from Failure to Success in Selling." Then, start acting enthusiastic. It just might change your life.