Dogs labour for humans just as much as other animals. They are bred to labour for us in various ways: as guards, entertainers, bodies for research, and as companions. Using an animal for companionship is just as much use as any other use and it is probably as companions that dogs endure the most cruelty. Cruelty and neglect of their well-being is rampant. Greed, ignorance, laziness, cultural beliefs, and incompetence due to age or addictions or low intelligence, are the reasons companion dogs suffer in ways that make the abuses other animal species endure almost pale in comparison. Dogs are man's best friend and therefore are abused by mankind more than any other creature. It is in the perverse nature of human nature to abuse what it best loves. Not all humans of course, but too many.
All dogs, including companion dogs, deserve to be free of their enslavement to our wants and needs. They don't exist for our purposes anymore than does any other species of animal.