Enthusiastic appreciation for something is more than just liking it — it's loving it. You're an enthusiastic fan of Italian food if you not only like it, you can't live without it.
Enthusiastic's Greek origins paint a vivid picture of how an enthusiastic person got that way, as the word literally means "possessed by a god." You may recognize the "god" part of the word — theos, as in theology — "the study of God." If you're enthusiastic about working on your vocabulary words, you don't have to be reminded to do it — you can't wait.