I went to YA Highway and saw this question posted. I think it's one that I often ask myself when I am staring at a blank Word document on my laptop. What am I going to write about? Why should I write about this thing instead of the other... I am always confused, but I often find myself writing along the same theme- horror or thriller. I think I am influenced by the things that excite and entice me, and also by the things that I think will never happen to me. I get to live these experiences and thoughts vicariously through the characters that I create.
And then there are the rare times where I just write and a story pulls itself together- it's often not very good, but it's a start!