Since the birth of the internet, breeding dogs has become a very profitable business. If you go back over 20 years ago there were about ¼ of the amount of breeders than there are today. The internet has allowed for the sale of dogs not only to be National but also International. Most Breeders today are not reputable. They are Backyard Breeders and Puppy mills. These people do not take any pride in breeding their dogs nor do they have any integrity or compassion. Most of them do not check bloodlines for disease nor do they vaccinate the Mamas or their pups nor get them proper medical care. Most of these dogs are never let out of a cage. They treat them inhumanely with hostility and cruelty.
As a result of these actions, the Mama has developed a fear and distrust of people. Unlike us humans, dogs start teaching their pups from birth. So what is Mama teaching these pups? She is teaching them to be distrusting and fearful of people. Also, by mistreating the pups especially during the imprint stage will cause a negative response and behavior pattern towards people. The pups with a Fight response can develop aggression if this behavior is not corrected. The pups with a Flight response will exhibit their fear in a different manner by cowering, running, and hiding.
These dogs and pups are a product of neither their environment to no fault of their own nor their genetics.
Certain standards need to be implemented for Breeders and consumers need to be educated in order to put an end to Puppy mills as Pet stores usually acquire their dogs from Puppy mills. This is one of the main causes of dog aggression.