1. People acquire cats and dogs from sources that perpetuate overpopulation. They buy from breeders and pet stores. 5,000 puppy mills breed more than 1/2 million dogs in deplorable conditions, most of which are shipped to pet stores. By buying from these sources we perpetuate the market. 25% of the animals in shelters are purebred. When a pet is purchased, a shelter animal dies.
2. Many pet owners who fail to spay and neuter their animals. Whether by intent or neglect, these dogs and cats reproduce. For every litter born, that many will die in a shelter. Many of these people are hobby and professional breeders, others are backyard breeders wanting their children to see the miracle of birth. Perhaps these people need to visit the killing room of a shelter to watch the tragedy of death.
3. People treat pets as disposable commodities. People simply surrender or abandon their animals instead of taking responsibility for them for their lifetime. Every day people walk into shelters and surrender animals for reasons as frivolous as the animal not matching their furniture anymore or we're going on vacation and can't afford to board - we'll just get another dog when we come home. The number one excuse is moving - the owner doesn't consider the animal important enough to find animal friendly housing. More excuses include - behavior problems, not enough time, can't afford, and allergies.

In summary, failure to spay and neuter is the major cause, but how we acquire our dogs and cats and how long we keep them is also a contributing cause. For every animal killed - be it a dog, cat, pig, rabbit, or bird - there is a human outside the walls of the shelter responsible.