The reason for dogs to exist is simple …
Everyone needs at least one creature that will look at you and think you are the single most important thing in the universe … that depends on you for food, water, love … that is eager to do anything so that they can get approval …
The reason for cats to exist is also simple …
Everyone needs at least one creature that has no such illusions … that will look at you and see you for exactly how very unimportant you are in the scheme of things …