I personally can't imagine owning just one dog.

Because I have a busy schedule, I do not feel I have enough time to devote to a single dog. I think that the single dog's people become their pack and playmates to a greater degree that they do for multiple dogs. This can be either a good thing or a bad thing.

My friend Bob, for example, has one dog. That's the way he wants it, because it's just the two of them and he wants Jack to act more like a human and less like a dog. He has the time to devote to his dog.

I, however, don't feel that I have that time, and I want my dogs to have canine siblings to play with when I'm not available. I want my dogs to be less needy, and more self-sufficient.

I've never really had dogs learn bad habits from new dogs, usually the new dogs learn the good habits from the old dogs, for the most part. The notable exception ... males and marking. I've had my dogs pick up marking from foster dogs.