Imagine for a moment a tailor selecting a patterned fabric and cutting out just the right piece. What he leaves out of the cut is as important as what he leaves in. Good photographers do the same thing. They look at the edges of their viewfinders to determine the outline of their "cut." However, sometimes the subject matter is so interesting that one forgets about all this and just shoots the photo. No artifice, just a quick draw of the camera. The image is all there for you, no assembly necessary.
This photograph is one of those cases. When I saw the woman in New York with the Great Dane at her side and the little Chihuahua in her arms, I rushed to aim the camera and shoot before she could walk by me. So there's no fancy composition, just a fun image that stands by itself. That's the beauty of this medium. A good photo sometimes just walks right toward you.