It's not too different than other hobby turn professionals. Like to work on cars? You may want to become a mechanic. Replacing brakes for the hundredth time is not nearly as interesting or challenging as the first time. On the upside, you get to be around the environment you love and occasionally there may be some new problems to solve. As with any profession, there is always an opportunity to learn and improve even on something that is considered routine. For a mechanic, maybe do it faster or with different tools.
I think "mechanic" is a bad metaphor. We're talking about engineering here. I don't think I could doubt that a lot of programming jobs (webapps, etc) are not as interesting as a lot of electrical and mechanical engineering jobs. But I could be wrong.
