The socialisation period from 3-14 weeks is the most important developmental window in a dog's life. What puppies experience during this time shapes their behaviour and welfare for their entire lives.
Under-socialised dogs live in a state of chronic fear and anxiety. Novel stimuli — unfamiliar people, vehicles, sounds — trigger stress responses that are frightening and exhausting. These dogs often cannot be taken to public places, experience vet visits as traumatic, and may respond to fear with aggression. The welfare cost of poor early socialisation is paid throughout a dog's lifetime — sometimes 15+ years of chronic anxiety from a failure in the first 14 weeks. Good socialisation is the single highest-return welfare investment available to puppy owners and breeders.