Writing a book is a laborious, lengthy process. I feel strongly that if I, as the writer, don't stay interested in the work, neither will any reader. So I pick topics that fascinate me or trouble me or captivate me, topics that will stay interesting even as I proceed to examine them from many different directions. Although I don't intend to make political or social statements with my work, the exploration of controversial topics usually involves the precipitation of polarizing conflicts between characters. From polarization inevitably comes point of view.
Thrillers are good stories that take readers for a ride.
I like to learn something along the way as I'm writing, and I like to offer readers an opportunity to learn something along the way if they are interested in doing so. I like to think each book is about something. Perhaps ignorant, or naive, is a better description. But I also never gave a moment's thought to the profound attachment that publishers and readers develop for continuing series characters. I'm not sure I should be admitting this, but until I begin a new book, I have little or no idea what is going to happen in the lives of the series characters.