[Betsy Iole]

Puppies need socialization, and puppy kindergarten provides an excellent socialization opportunity. Socialization produces a calm, well-behaved dog that doesn't react unpredictably when confronted with novel situations. The window for optimal socialization closes quite early, around 5 months or so.

Puppies who never see the world beyond their immediate families and backyard, except for trips to the vet, are at significant risk from being poorly socialized. From Early Socialization

Dogs that are isolated during their first year of life develop many problems. A few will become aggressive, but the majority are more likely to become overly shy or timid. They lack confidence in the presence of new people or situations. They cower in the presence of strangers. They jerk at their leash to get away from children or other pets. Forced to be in a new place, they may sit shaking behind you, drooling, and panting rapidly. In the worst case scenario, they may become fear biters. This is a behavioral pattern in which dogs, when encountering new people or pets they are afraid of, do not know how to react and simply attempt to bite the stranger. Once this develops, it can be very difficult to overcome.[/b]

Additional reading
Socialization Tips and Training Pointers for New Puppy Owners
Puppy Socialization: What It Is, Why It's Essential, and How to Do It