I hear this a lot too.. “My puppy is well socialized.”
– “Because I have 4 other dogs.”
– “Because we have 3 kids.”
– “Because it’s Christmas and the family came over.”
– “Because we go to the park.”
– “Well, he will be after his vaccinations. We’ll walk at the park so he sees other dogs.”
– “We’ve done dog socialization but he still needs people socialization.”
– “He’s well socialized with people, but still needs dog socialization.”
At the Midwest Veterinary Conference last year, Lore Haug CABC, CPDT, DACVB, MS said something I thought was especially clever. Good socialization is about positive experiences, “not bad, and NOT neutral!” And that really is a key piece with puppies. Just because you walked through a busy park and your puppy was fine, your puppy was having a neutral experience. We want that extra bit of food and play and attention that makes it a good experience!
Puppy socialization is never “done.” You can accumulate more good experiences and the more, the better! Dog and people socialization is not about walking past. But having good interactions. It’s not about meeting a few or even 25 people. It’s about a lot, and variety (and good experiences!).
But the part that makes me very sad is that while the public has a growing understanding of puppy socialization, the common view is that this is about people and dogs. We need to have good experiences with handling, noises, smells, sights, sounds, surfaces, interactions, new environments… people and dogs are just components of that. This other stuff is super important while a puppy is developing!
Puppies are a lot of work. Last spring, I was training two golden retriever puppies. They were about 16 weeks old, well past their primary socialization period and had never been outside of the kennel where they were born. Most of the ‘training’ time was spent on everything in that above list, trying to teach the puppies to be brave and love the world and be curious and to just have a ton of great interactions with the world. We would walk in town many times per week. A person walking past? Treat. A dog walking up? Treat. A car driving by? Treat. A skateboard! Treat. A statue. Treat. Shouting. A treat. Kids running. A treat. They’re doing quite well now.
And our adult dogs need ongoing socialization, but not necessarily the extent that puppies do. Shy dogs especially need to be out and about having good (not bad, not neutral!) experiences with the world. And many shy dogs tend to regress if you don’t keep up with their work.
Griffin walked up, and the puppy rolled over. Tail tucked. And peed. He was a puppy that did become a normal puppy after having good, not neutral, experiences with the world. 
Griffin: “Are you okay? Did I say something?”