Sometimes as you work, you find that you are learning things about your own perceptions and motivations that are way below you consciousness. If you get lucky, you recognize what you are doing, but all too often we don't find the connection between our work and our own motivations.
You see shape, and how the light hits things, how the color changes from one end of the photo to the other, and how movement affects the mood of the photo.
As you see something that yo want to shot and it's bearing down on you, it's important to start framing long before the subject gets close to you. The light will reveal itself possibly long before you want to take the image, but you have to wait until the picture comes to you, and if you've been anticipating carefully when the subject will be in position, the background will have been figured out in advance.