If you take a young puppy and place the dog in front of a mirror. That pup will bark at his own reflection thinking it is another dog. In the pup's mind, it is saying, "Wow! I found a new playmate." But after futile gestures for the pup in the mirror to interact, the pup eventually loses interest. The pup eventually may grow up and ignore the mirror only seeing it as a useless object with no purpose.
There are however animals with self-awareness which will recognise themself in a mirror. Animals such as Dolphins, Orangutans and Gorillas. Most other species of animals such as cats, dogs, raccoons, bears etc, will either treat the image as another animal, or come to ignore it completely.
Gordon Gallup, a psychologist from the State University of New York at Albany, did a similar experiment on chimpanzees. First he introduced a mirror into the home cage of a chimpanzee. At first they reacted as if they were seeing another individual but over time they learned that this was their own reflection. Next Gallup anesthetised the chimpanzee and painted a red mark on its eyebrow and another over its ear. When the anaesthesia wore off, the chimp failed to show any interest in the marks until it caught sight of itself in the mirror. On seeing its image with the red marks the chimp began to act like children who know that they are looking at themselves in the mirror, and began to touch their own eyebrow and ear, while carefully watching its image in the mirror. Gallup believes that this means that the chimp is self-aware. It understands that it is an individual and that the reflection that it is looking at is of himself.
The conclusion that researchers drew from the fact that dogs fail the mark and mirror test is that dogs lack self-awareness, and thus consciousness. Another conclusion that could be drawn, of course, is that dogs recognize that that is their own reflection, but they are simply not as vain and concerned with their appearance as higher primates.