Your camera is basically a single eye — no depth perception other than depth of field. Your images are 2-dimensional. When you look through your own eyes, you see the world in 3-dimensions.
This can cause you to see things slightly different than you would through the camera. Have you ever picked out your subject, taken a shot, then realized that it didn't look quite the same as you remembered it? Your 3-dimensional vision can cause you to see and focus (mentally) on different things than your camera is capable of picking up.
So to help bring yourself a little closer to your camera's point of view, close one eye when you think you have a good shot scoped out. Evaluate the colors, tones, shapes, contrasts, etc. THEN evaluate through the camera's viewfinder for framing and composition. I find myself doing this little exercise even when I don't have a camera on me.