I write about Linux personally just because it is an interest of mine and plays a big part in my day-to-day life.
Writing about something we're passionate about is definitely the way to go... otherwise we would tire of updating our blogs really quickly!
you have said right, when we write about something which is very much favorable topic of our interest we dont need to think about the blog updating, on the other hand if the writing is about a boring topic which we dont like, the blog remains incomplete with old posts.
Please view this thread about "Blogging For a Living"... there are some really basic but useful tips here... http://forums.digitalpoint.com/showthread.php?t=1953971