I agree about the companionship and the life span. I have had many animals over my life. You can't really hug a fish and it's kind of boring watching them swim back and forth (at least to me). Hamsters are adorable, but they have such a short life span.
You can bond with a dog or cat in a way that you can't with other animals. You can still love the other animals as much. It's just that the reciprocity isn't quite there. You get a lot back from a dog or cat.
Of course, we know that many people have bonded with different kinds of animals. So, there may be other factors. With dogs, it could be that we take them everywhere we go. We take them for walks, we like to bring them to family get togethers, and so forth. They become another family member. Cats aren't with us quite as much, but they do have free reign of the house. They are following us around while we go about our routine or sitting with us when we watch TV. Maybe that is part of it. Dogs and cats are not caged pets, so they are with us far more than the other pets that we may have. Their involvement in our daily lives is at a different level.
