Urine-Marking: Why Dogs Mark Their Territory

Source: 

The Humane Society of the United States

Much like the miners during the Gold Rush, dogs are territorial animals. They "stake a claim" to a particular space, area, or object by marking it, using a variety of methods at different levels of intensity. For example, a dog may bark to drive away what he perceives to be intruders in his territory. Some dogs may go to the extreme of urinating or defecating on something to say "mine!."