Hm...I honestly think that if I didn't go to grad school to research my topic of interest, and instead took some unrelated career path, I would probably still spend free time reading books and articles on the topic (I wouldn't write papers or anything, let's be realistic) because it's truly my passion. Might as well try to get paid to do something I'm that passionate about!
I know that my topic of interest is a complex problem that will never be solved completely, but knowing that I could actively contribute to efforts as a career is perfect for me. My research interests are also applied, so it would feel good to "help".
Also, I would never be happy in a career that didn't ask me to solve problems. In psychology we call this a "need for cognition", and I assume essentially everyone in these forums has a high need for cognition.