I consider a hobby as something that occupies my time more than it really has to. For example, I am a cigar smoker and have been on several forums about them. You get into things like storing cigars, getting the best deals on great cigars, and learning the best way to cut and light the cigar. It's pretty much endless because you want to learn everything you can about what you enjoy.
With a coal stove for me, I had no experience using one at all so at first learning as much as I can is out of necessity. There are many aspects about burning coal which are interesting and make it into a hobby of sorts, but while cigars make me happy, coal keeps me and my family warm, so it has an added benifit.
Now, the logging on to the site a couple times daily, and scrounging around the area looking at old coal stoves, and browsing the internet for the "dream stove" starts to get into the hobby mode, and borders on the obsession/fetish, but I don't see myself "loving" my stove anytime soon. OUCH!