Enthusiasm, like optimism, is a choice rather something that just happens to you. It requires a lot of forethought (what do I really care about?) and continual maintenance. You need to spend time discovering and remembering the fascinating parts of whatever you're doing, and the end-results you're shooting for.
I was stuck in an awful job stocking a supermarket, with no alternatives I could find. I realized I had a choice about how I approached it, I could either mope around all day or find something I liked about it. I discovered my OCD side really appreciated looking down a whole aisle and seeing every can perfectly stacked with the labels aligned. It still makes me smile thinking about it, and left me enthusiastic enough to stick with that job for years, get through college and move onto other goals.