I’ll admit that this is something that I haven’t really thought of before, but after having watched this excellent video from aows, I think it explains why I often change the topics I like to shoot. His argument is that the more you do photography, the more images you shoot, and the better you become, the more difficult it becomes. It seems odd, but when you think about it, it makes a lot of sense.
The reason it gets harder is that you’re following that advice often told to photographers. Stop competing with other photographers. The only photographer you need to compete with is the photographer you used to be. And this is basically the problem. As you learn more, get better and your work improves, you’re setting your own bar higher. It gets more and more difficult to best yourself.