Young puppies are innately curious about their environment and will investigate new objects and environments. Puppies less than sixteen weeks of age tend to stay close to their owners and trust that their owners will protect them from danger while they explore. As puppies become more independent, they also become less willing to explore their environment and start to fear anything new. This is actually a highly adaptive trait as puppies that are too bold and stray away from safety are likely to be hurt. It is important that you expose your puppy to as many positive experiences in different situations as possible before your puppy is five months old.
Puppies that have not been adequately socialized; i.e. taught correct behavior, around categories of people (men versus women, adults versus children, different ethnic groups) during critical periods of development will be fearful of these individuals when they grow up. Puppies that have not been adequately socialized around other dogs will often be aggressive to other dogs as they grow up. Fearful behavior will also occur in situations that a dog was not exposed to as a puppy.