In puppy classes, we help build puppies' confidence and experience by
giving them opportunities to interact with different obstacles and surfaces.
Older dogs can obviously still learn to accept new and potentially scary things, but the process typically takes longer and can be more difficult, so part of laying the foundations of a lifelong relationship with a well-balanced dog is giving a puppy lots of positive, mellow experiences with the kinds of things he'll need to be comfortable with throughout his life.
That means puppies need to meet people of all kinds of shapes, sizes, dress codes, faces, voices, and behaviors. An unsocialized adult dog might be fine with his family, but outside that zone of safety, he's more likely to be fearful or overexcited around unfamiliar things like glasses, hats, or beards if he's never learned that they aren't signs of danger. New stimuli can be very intimidating to dogs, and if they react with fear, that can lead to more than just an unhappy experience. Fear and other kinds of overstimulation can cause issues ranging from common problems like an inability to focus around distractions all the way to very dangerous behaviors, like running off or even fear biting.
Just a few weeks ago, I wrote about how context is king in a dog's world, and socialization can be understood as an extension of that concept. Any unfamiliar aspect of a situation can mean that the dog does not know what to do to create safety for himself. In the house, with people he knows, he has learned what he can do to feel safe, like where he can go to hide or what behaviors he can offer that get him positive attention, food, toys, and games.
But when the situation changes, even a little, the dog might not even know that the world is still safe and that humans are still going to be kind and predictable. Dogs are masterful at a lot of things, but they are not good at generalizing or inferring. As humans, we are so good at those things that we naturally take them for granted. For example, if your dog has never met a man with a beard, he may not be able to generalize that a man with a beard will behave in a similar fashion to all the men he's met without beards. To us, that's incredibly illogical. But to a dog, a man with a beard might seem like a brand new creature. It's like if you turned a corner and were confronted by a bear wearing a tuxedo.
So, for example, when a puppy sees his first adult in a ball cap, he should get a yummy treat, especially if it seemed to make him nervous. If he tries to get away from a new thing, like a trash can, even if his fear is completely unfounded, let him. Teach him that if he's upset, he'll be able to get away and then inspect the thing at his own pace, not that you'll drag him closer or force him into things. Teach him that any bravery he shows will be rewarded with calm praise and a treat or toy.
By sixteen weeks of age, a puppy should have done as much of the following as possible: met a hundred different calm, friendly people; played with a dozen different friendly, healthy, vaccinated dogs; walked on a dozen different kinds of surfaces; seen some of the local wildlife; and had a whole variety of other experiences you can come up with that mimic the kinds of things he'll run into in the life you're planning for him. Here's a printable checklist in PDF format that you can use to track your progress and make sure you haven't missed any of the common items people overlook when socializing their puppies. The checklist offers overall goals, like meeting 100 friendly strangers and walking on 15 different surfaces, and it also gives lots of specific items and a way to keep a record of how well the experience went so you know what kinds of things you may want to revisit.
One last thing: if you're reading this and your dog is older than sixteen weeks, that doesn't mean that the work of socialization is done. Maybe you didn't do everything the experts recommend before sixteen weeks, or maybe you have an adult dog with an unknown history. It doesn't matter now; socialization is a lifelong process. The early puppy window is really helpful, but every positive experience your dog has with a new thing is still socializing that dog to be more confident and stable. And if you socialized your pup by the book through week sixteen, then pat yourself on the back, but still remember that your work isn't over. You can still socialize a dog after he grows up, and early socialization isn't the only thing a dog needs. If you want a calm, confident companion dog, that's something you create opportunities to encourage wherever you can through a dog's whole life.
Sources: the St. Hubert's Animal Welfare Center puppy packet, the Paws N' Effect puppy packet and class curriculum, the AVSAB position statement on socialization, Socializing Your Puppy by the ASPCA, Dr. Sophia Yin's Checklist for Socialization, "Human directed aggression in domestic dogs" by Casey et al., Puppy's First Steps by Dodman et al.