You know, for me this is a mixture of both. I KNOW that I can make money writing on Hubpages. Sometimes I just get great ideas for Hubs, and I write them, then publish them without thinking "Oh how much money am I getting from these hubs". Then there are other times when I do think "How many hubs do I need to write in order to cash out this month? To pay bills etc..
I basically write because I love to, AND because I make money from it. But this isn't the only website I write for and make money from. So, for me writing is split between writing for fun, and for money.