puppy early exposure
A pup should not be removed from its mother and littermates before 8 weeks of age (THIS IS THE LAW IN COLORADO AND ALL OTHER 50 STATES). If you get a pup sooner, then you're wasting a lot of prime socialization building time with the pup's mother and pack. Yes, the pup may be capable of leaving at 6 weeks but if you want a dog-socialized pup, then it should stay with its litter for another 2 weeks to help with its dog social skills. That's when dog socializing goes on - during those weeks and with its littermates and other attending adult dogs, not with strange dogs from outside the pack during the next 2 months. There is no reason or need to trust other dogs at this point.
This leaves you with a total of 8 weeks that the puppy must be kept from other dogs (between 8 weeks and 16 weeks). This isn't a very long time to "ruin" a dog in terms of dog socialization. This, as a matter of fact, is the most important time in bonding with YOU, not other dogs. The nice side effect of keeping your pup away from other dogs at this point is that it makes you spend more direct time with the dog. If you feel that you absolutely must have the pup socialize with other dogs during this period, reunite the pup with its littermates, its dam, or other members of its direct pack. If this isn't possible, any other dogs in your own house will suffice. If you don't have other dogs, then you shouldn't go any further.
Pet fairs, dog events, contests, and doggie parks are the worst place to bring your pup before 16 weeks of age precisely because they have so many other dogs. I cringe every time I see a young one at any of these events. These are prime places for the communication of nasty diseases and increase the pup's exposure 1000-fold. (Remember what happened when one kid in school got chicken pox?). There is almost always no vaccination requirement for these events. Infectious diseases such as kennel cough and the like run rampant at these places. Ask regular pet-event travelers how many times their dogs have "picked up" some cough or diarrhea at one of these affairs. For an adult dog, kennel cough is merely a nuisance. For a young pup, it may be life-threatening
Just because any particular dog is vaccinated doesn't mean it isn't a carrier of a serious canine disease. The dog may be an asymptomatic carrier. Or, remember, disease agents can travel on, in, or with a dog, immunized or not.
An immunization prevents a dog from getting a disease - it doesn't prevent it from carrying it. My dog may be vaccinated for parvo but still may harbor it in its feces, intestinal tract, in its paws, on its coat, wherever. Vaccinated dogs tend to go everywhere - especially where other non-vaccinated dogs roam.
Your neighbor's dog may seem safe but when you consider that it goes down to the local park where all the non-vaccinated dogs wander, and defecate, and sneeze, etc. and then comes back to your house, your pup isn't safe at all.
Even people can bring parvo in on the bottom of their shoes or on their hands. This is why very few outside visitors should be allowed in the house when there is a litter present - you never know where they've been.
Many Trainers: Many trainers, the Monks of New Skete for example, point out that the dog's socialization with other dogs occurs in its litter and pack before you ever pick up the puppy. Any other dog socializing can go on at some later point.
Poorly dog-socialized dogs, even if they can't be rehabilitated need to be kept in single-dog homes, not euthanized. A people-aggressive dog is one thing and may call for euthanization - but a dog-aggressive one is something entirely different. A dog-aggressive dog can live a complete and happy life, without a single problem, in a single-dog household.
From the very early exposure of young puppies to changes in their daily living to the gradual introduction of human children and adults' interaction, tone of voice, play equipment to explore, the list becomes as varied as the imagination of the breeder. And from the breeder to the new home, experiences that broaden the life of the puppy should continue. This is the process of "socialization".
A responsible breeder will recognize that even the youngest of puppies needs to be exposed to varied experiences.