Socialization is the process of learning how to be a part of society. Socializing puppies involves helping them learn to be a pet within human society and dealing with different types of people, environments, noises, smells, sights, animals, and other dogs.
Puppies are most accepting of new experiences between the ages of 3 weeks and 12 weeks. Between 12 and 18 weeks of age, it becomes more difficult to socialize a puppy as they become more cautious of anything they have not yet encountered.
Well-socialized puppies are more comfortable with a wider variety of environments and situations and are less likely to act fearfully or aggressively when they encounter unfamiliar places, people, or dogs. Well-socialized puppies usually develop into safer, more relaxed and enjoyable pets and can live more peaceful, happy lives because they are not stressed out by new situations.
Socialization requires time and effort. Exposing your young pup to different things they will encounter in their routine life can help the process. Depending on the lifestyle you have planned for your dog, you might include exposing your pet to the sight and sound of trains, children, garbage trucks, crowds, cats, or crying infants. While it may be impossible to introduce your puppy to all of these situations during the peak socialization period of 3 to 12 weeks, the puppy may be more likely to generalize past experiences and find something familiar in a new situation. It is important to get any type of dog used to the experiences they will routinely encounter, including the types of people, sights, sounds, and physical handling that will take place in their daily lives.
— Janni Kimble, RiverWoods Pet Hospital