Peter -
I suspect that if you were to take the time to carefully study your work, concentrating on the images that you especially enjoy, you would perceive that there really are several distinguishable themes present. I know that in my work, I can identify themes retrospectively, but there's no way I would ever have recognized a conscious intention to go out to make images on those themes. Instead, as I go from place to place, I react to the scenes that present themselves to me, and make photographs of the ones that I like. There is a selection process involved in the decision to make an image, one that I am not aways actually aware of at the time.
I know who you were with last week, and when I was in his workshop a couple of years ago, we were sitting around one evening have a beer or three when he said something like "there's an image over there - - - and the pain of not capturing that image on film has now exceeded the pain that will be involved in setting up the camera to photograph it", whereupon he set up his 11x14 and made an exposure. The point is that the image suggested itself to him - he did not seek out the image.
That said, I know that there are certain things that I enjoy photographing more than others, and I know that there are also some subjects that are very popular with other photographers that I don't especially enjoy. That means that I probably do bias the process of images "finding me" by more frequently putting myself into those settings that are more likely to present the kind of images that I enjoy. For example, I don't get very turned on by "roots and rocks", so you won't find me hiking up mountains. But I do like manmade things, so I tend to gravitate toward situations involving buildings, structures and machines.
Colehogan raises an important point - many of us have been (and still are) involved in camera clubs. I know that I have learned a lot from the club experience, and that the inspiration of the club has challenged me to improve my photography. But as one matures as a photographer, there is a point beyond which a club can be a problem. Clubs establish expectations, and satisfying those expectations can limit further growth of photographic vision.