If there’s a book you really want to read, but it hasn’t been written yet, then you must write it.
This kind of says it all for me; I write what I’d like to read, what interests me, not what “the market” says is hot. If I can’t get excited about writing a particular type of story, how in the world am I going to keep a reader interested in reading that same story? My disinterest or discomfort is sure to show.
At the same time, I think of what I’d love to read, what makes my imagination snap and whine with eager anticipation…and I write that (or at least I try). Here my enthusiasm shines through the words on the page, helps to carry the reader along on the ride with me. Enthusiasm is contagious. Love what you write and you’re a step ahead of the game.