Notice that I am not asking, why do you have to write?
The answer to that may be prosaic: I have an assignment, or I want to get a promotion, or I need to earn a degree. Or more dramatic: because I have an inner daemon, because it is my gift, and so on
My question has a very different focus. I want to know why you feel compelled to write about a certain subject. What ideas rattle around your head at night and make you so restless that you get up at 3 in the morning (remember that's it's always 3 A.M. in the Dark Night of the Soul) and put pencil to paper or turn on the computer? In other words, what about your writing really stirs your juices?