Fearfulness

Socializing a puppy to people is the easiest and most enjoyable aspect of raising a dog. On a regular and ongoing basis, puppies need to meet, play with, and be handled and trained by a wide variety of people, especially including strangers, men, and children.

Narrow Window of Opportunity

Old dogs can indeed be taught new tricks. An adult dog may learn basic manners and good behavior (where to eliminate, what to chew, and when and for how long to bark) at any time in his life. However, socialization must occur during puppyhood—during the critical period of socialization, which ends when puppies are 12–13 weeks old. Shy and fearful dogs can be substantially rehabilitated, but they will never develop the confidence and social savvy of a well-socialized puppy. They will never become what they could have been.