I think the key is to realize that, once the outline is finished, you only think you know the story. There's a lot of new, interesting stuff that evolves or gets invented when you're connecting the dots between outline points. You just have to dive into that "fleshing out" phase with the understanding that your story isn't completely told, and there's still a lot for you to discover. The only difference is that you're switching gears from brainstorming forward to brainstorming between. And in successively more detail.
As others have pointed out, though, even if that idea doesn't excite you right off the bat, you'll need to learn to push through those "I don't want to" moments if you ever hope to be a pro. A HUGE part of writing is rewriting, which means even the step you're avoiding right now is only an early step. A first draft is just the beginning, not the end result. Whether you discover an enjoyment of the process or simply discipline yourself into continuing forward, you'll need to find a way to push through many, many stages of writing. If you can't, you might as well start shopping for another career now.
That being said, I think you'll find that if you just push yourself forward and start the next step of your writing, you'll very quickly find the fun in it. After each major phase of a project (outline, treatment, first draft, etc), I feel like I'm done and have no interest in continuing. I want desperately to procrastinate and do something else or start another new, interesting project. But, as soon as I force myself to start the next phase, I very quickly get into it and start having tons of fun, and wonder what the heck I was avoiding in the first place. If you're truly a writer, I imagine you'll find the very same thing.
So, yeah. Just force yourself forward, and the fun will come. The more times you find the fun, the easier it'll be to force yourself forward the next time. Rinse and repeat.