It is definitely more of a passion. For me, I started out thinking, hey! good idea to make a little extra money! Seriously though, I don't even really look at that anymore, it is nice, but it is more of just a little bonus than anything. I enjoy writing on hubpages, much more than I originally expected to. I have a goal of 1000 hubs, currently I am only at 31, I am far from it, but my goal is for the simple fact that, as I write, I also learn more and Knowledge is power! I think it is the same for a lot of people. It starts out as a money maker, and turns into a passion without you really realizing it, it just slowly progresses into a burning desire to "hub" the more you do..
Thanks for the question! It is definitely something to reflect upon for some people!