Up until they are 8 weeks old, your dog is surrounded by other puppies and their mother--spending his days playing and learning with his littermates. Then they we sold or adopted out, and they leave their social circle and are bought into a life that is totally different than litter life before this!
So often puppies are brought into a zero dog household to be a friend for the kids or simply a companion for their owner--which is wonderful! Sometimes, it's even a household who has a cat and wants to add a dog to complete the circuit (you know I love kitties, so I love that type of family!) Or maybe it's a household with a small dog, where they want to bring in a bigger dog to have a variety! Whatever the case may be, it is important that your dog socialize with others outside of your household.
A cat can be a fun friend for a dog, but it has different body language and social signals. Kids are great buddies for a dog, but have sensitive skin that will be bruised and scratched with rough play. Little dogs are definitely an important part of life to be exposed to, but a Great Dane can't play the same way with a Chihuahua that it would with with another larger dog.
Dogs are social creatures, and as much as we strive to be the best leaders and teachers for our dogs as we can, ultimately other dogs are truly the best teachers out there. If we keep socializing our young dogs with other well behaved, balanced dogs of all ages, they will grow up to continue to be well socialized adult dogs! That means they can read body language, greet other dogs appropriately, and know how to communicate through subtle signals with each other.
I encourage you to spend the extra money on doggie daycare a couple times a week, meet with your friend's well behaved dogs, or just get out and walk your dog in a busy neighborhood--and do it often. We do no favors keeping them cooped up behind our doors, or trapped to the patch of grass known as our back yard. Keep life exciting and keep it social--your dog will love it and through his blossoming social behavior, thank you for it :)