How do you choose what to photograph?

I photograph what presents itself to me and what appeals to me.

First I see the overall picture
Then I decide if it needs foreground interest or not.
If it does need foreground interest I look around for rocks, plants, lead-ins from the lower part of the shot to the main area
Decide on camera height
Choose which filter to use, if any.
Check for and remove distractions from the image, especially at the edges. A tripod is really helpful for this.
Check camera settings to make sure the image will match your expectations; use a small aperture like f22 if you want everything in focus, use a fast shutter speed of at least 1/100 sec to freeze water droplets from a waterfall in mid-air.
I also take photographs that people ask me to or to fill gaps in my library after receiving requests.