Puppy owners do not recognize the VITAL importance of proper puppy raising in the first year of life.  Proper experiences or socialization for the first 12 months will essentially "make or break" that dog's ultimate ability to become a fully functional adult.
 
It is imperative that you make every effort to expand the puppy’s environment and expose him to new things. During the first 12 months of their lives, puppies should be around as many different people, places, and animals as possible. Take them with you when you go for a walk, shopping, or even to work. Encourage your children to bring their friends over to meet their new pet. Its also imperative that you take your puppy to obedience or training course where they will meet other dogs in order to learn doggie manners. Obedience courses are also the ideal place for the inexperienced owner to learn about basic training, and to work with a qualified trainer who will help guide you with training advice for any behavioral problems that may arise.
 
Many dog owners believe that their dog's fearful behavior toward strangers is from someone mistreating or abusing the dog in the past. In reality it is a lack of proper and extensive social contact with the world. That is why every puppy book available repeats itself over and over " take your dog everywhere, let him meet everyone young, old, male, female"
 
Great Socialization links:

http://www.paw-rescue.org/PAW/PETTIPS/DogTip_SociablePups.html  

 
DIFFERENT STAGES OF PUPPY DEVELOPMENT

During the first 2 yrs of life puppies go through many stages of mental development, so it is important to understand these stages as they occur.  Many adolescent dogs are turned into shelters, euthanized, or given away because the owner has not been educated in the behavioral development of their puppy. Understanding these stages, and working through them with your puppy will make for a wonderful family companion when he reaches maturity. Scientific studies have shown that there are FIVE critical periods in a puppy's life. That is five phases of mental development during which adverse conditions could cripple a dog emotionally for life. Conversely, positive conditions during these five phases, will produce dogs of the highest calibre mentally, and socially. If at any time you find you are having problems during any of these stages, please contact your breeder or trainer, so that they can provide you with advice, guidance and support. Just remember that not all puppies/young dogs go the through the same stages, and each puppy will differ in its developmental stages.