Pet shop puppies are usually supplied by large puppy mills, with no early socialization toward humans, no training, and most often poorly fed. Buying one of these puppies only helps to perpetuate this sad state of affairs. Responsible breeders do not sell puppies through pet stores.
If you've never seen the deplorable conditions found in puppy mills or read about the horrible mistreatment of the parent dogs in puppy mills, please click on the links below to learn more.
Jan 2010 - 99 Dog Puppy Mill Rescue
http://www.youtube.com/watch?v=Svvdw5uKkaQ
51 dogs rescued from rural Puppy Mill
http://www.youtube.com/watch?v=vddWx7XsC0g
Gov. Schwarzenegger Duped into Buying a Puppy Mill Dog from Elite Animals - CAPS Investigation
http://www.youtube.com/watch?v=flDK74ZkURk