Taken from stoppuppymills.org: When we buy a pet or even shop at a store that sells puppies, we contribute to a heartless underground industry that forces dogs to spend their entire lives in cages constantly breeding to support consumer demand for puppies.
This video by Charlize Theron, explains a lot:
Please THINK about the big picture before you purchase that seemingly cute little puppy. The more people who support this way of "buying" pets, the more they will be forced to multiply in horrible conditions. Instead, think about all the abandoned pets that need homes.
Pets purchased from a store tend to be impulse buys. Failing to consider all the ways this precious pet will impact your life can lead to behavioral and other problems. The worst part is the poor health quality of these pups. Working at the emergency clinic, I have personally witnessed everything from Parvo to hip dysplasia to seizures, and worse. These are serious problems, forcing their owners to make serious choices. Be responsible and adopt from your local shelter.