I interviewed the well known UK dog trainer
Jan Fennell a couple of years ago. I asked her about this and her opinion is:
dogs (and cats) are psychologically a lot happier with a companion, especially dogs that get little other opportunity to interact with other dogs. It allows them to relate to each other as dogs in ways we can never duplicate with them. A dog that is TOO focused on its owner is not a psychologically healthy or confident dog and will be likely to have separation anxiety (often unknowingly encouraged by an owner) and often develops problems dealing with other dogs it meets (too shy or too aggressive or indifferent because it doesn't know how to relate comfortably to its own species).
Speaking as someone with four dogs now, I can also say no dog's focus will lapse from you and go to a companion dog in any but a beneficial way -- I found a companion made it EASIER to leave my dogs alone, easier to have them exercise through play, easier on me to not have my every action the sole focus of their lives. They are all loving to me, but they are not the equivalent of the overly dependent mommy's child (which is perhaps a way of thinking about dog ownership. Your dog should be able to have an independent confident life without relying on your constant presence, approval, and reassurance. A dog that cannot function independently is not a psychologically healthy dog -- it will be flooded with anxiety all day long at any change in the environment and can become destructive out of pure anxiety if you disappear now and then to lead your own normal life socially or at work).
I think it is wise to consider the dog's well-being, if anyone feels they need to be the sole focus of their dog's attention -- this isn't very healthy for the dog. I know most people are not talking about an overly dependent relationship and simply mean they enjoy the loving equal relationship they have with their dog and that is normal -- but I think it is good to keep an eye on our own healthy relationship with our dogs to make sure we are not being too needy with them and thus making them too needy and dependent on us. It is not healthy for us or for them to place the burden of being everything to your dog on your dog to the point where we'd would feel jealous if that attention goes elsewhere sometimes (whether that be to a person or another dog), just as it isn't healthy to expect a person to be that overly focused.
I also think singletons can get along fine but keep in mind that such dogs need time with you and time with other dogs -- often they are left alone all day while people are at work and never interact with other dogs except maybe a brief greeting on a walk which again, is a fairly psychologically bereft life for a dog (this is actually why I think a stay in kennels is actually very beneficial for many dogs who may find that's the only chance they ever get to play with other dogs!). I also think no one should get a dog only for the other dog -- just as no one should have a baby simply to provide a sibling to an existing child! The responsibility extends to the entire human family to make that second (or third or whatever!) dog as much a part of the total family as the first dog. The second dog needs to be wanted in its own right, not be just a dog accessory for another dog.
Also dogs relate in all different types of ways. Some are physically close, others aren't. Some play together, others don't. Just like people, not all dogs need constant close activity with other dogs to enjoy and benefit from companionship. Getting a second dog means not getting one to fulfil the imagined relationship you want your dogs to have together, but to allow them the relationship they choose.
And finally: some dogs will not get along, a risk that can be higher between siblings homed together, and more so, between dogs of the same sex. So for the best results in adding a second dog, don't home two puppies from the same litter together, and get a dog of the opposite sex to the one you have. Cavaliers all tend to get along anyway, but the problems I have seen have almost all been between dogs of the same sex, or two siblings either too closely bonded and poorly trained as a result or that become competitively aggressive as they mature -- so I always recommend adding a different sex for the best chance of success.
