I have finally figured out, in less than a million words, what my Great Themes / Obsessions are. Sadly, they seem less great when boiled down thusly, but here I go:
The thing I always seem to be writing about, the thing that moves me to the page and informs every word, is love and compromise -- how men and women (and men and men, and women and women) negotiate intimacy, and how much compromising they do to pull it off, and how you figure out how much compromise is bearable/worth it, and how much is too much, and whether or not you can/should live with the way the other person changes you.
At least this is a minor improvement on my usual answer when people ask me what I write about: "Oh, well, you know. Love and stuff."