Your puppy needs careful, positive exposure to all the things that he might encounter as an adult dog.  A poorly socialized puppy can become an unfriendly or fearful adult dog.

A puppy’s brain is fully formed by five months.  Social interactions are especially important during the first year of a dog’s life.  Make sure you continue socialization throughout your dog’s life.  The most important thing to remember is that you are conditioning your dog to enjoy strangers, other dogs, and animals.  You must manage the situation so that your pup is not overwhelmed or traumatized.

Unfamiliar Situations

Take your puppy to safe places, and expose him to positive interactions with people or other dogs several times a week.  Puppies need to be exposed to many different people in order to generalize the concept that people are okay.  If your puppy looks a bit fearful (closed mouth, rounded eyes, flicking tongue, yawning or turning away to avoid contact), ask the people to stand still and let the puppy approach them.  Have strangers toss treats for a shy puppy, so he learns that people are wonderful.

Your puppy will also need to learn that being approached by unfamiliar dogs is a fun experience.  Interactions should all be with friendly dogs so your puppy learns to trust them, not be frightened by them.  Even if you own other dogs your puppy needs to be socialized with unfamiliar dogs, and needs to interact with them in order to learn all the rules of polite doggy behaviour.

Environmental Stimulation

Environmental stimulation is equally important to puppies.  Your puppy needs to
experience a wide range of environments.  These early experiences will help to create a dog that is more flexible about changes when he is grown up.  Take your puppy places where he won’t be scared by too much activity or noise; rides in the car, visits to the vet’s office (waiting room, with tons of treats), a friend’s house, walking in the woods, walking on shiny floors.  Go a wide variety of places but don’t allow the puppy to become overwhelmed by too much noise or movement.