Working as a developer full-time can be very exciting. Dealing with new interesting problems gets me up in the morning. But I still find that after a few months of a specific project I find myself less and less interested with it. Not only projects but concepts and languages start to become less interesting. First it was websites then Objective-C then Ruby then C and now who knows. This scares me. At this point in my life I hope to work in this field for my 'career' meaning a significant amount of time. Yet I can't even keep myself working on a single project now, much less one that doesn't interest me.
I typically blame this on the difficulty of the project. As a lone developer I find that most projects I work on are pretty small in scope, since I just don't have the time or people-power to work on anything larger. Therefore I look at most of my projects without interest. I'm not sure what it will take to keep myself engaged and interested on my next project.