Claire Berlinski:

Every year in America, five million cats and dogs are gassed to death or lethally injected with sodium pentobarbital in these shelters. The word 'euthanasia' is a grotesque euphemism. There is no mercy in these deaths. Most of the animals are healthy, rambunctious, and young. They die terrified, and they die pointlessly: very few are vicious; most are capable of forming deep affectionate bonds with humans. This is what happens -- what really happens -- every day in these shelters. The links are graphic and upsetting. They're also reality.

Concern for the welfare and dignity of animals is not confined to nihilist Leftists such as Peter Singer or local totalitarians who seek to regulate pets out of existence. Have you read Matthew Scully's immensely moving, immensely disturbing book Dominion? A completely conservative case can be made, should be made, for treating animals with mercy and respect. Animals are not ordinary commodities, they are living creatures, and they feel pain and fear. No one need suggest that a kitten's life is morally equivalent to a human's to observe that something is terribly wrong when we casually dispose of one much as we would the butane in a Bic lighter: that is the mark of a callow society, a cruel society. It does not speak well for us that we kill millions of sentient, sensitive animals every year through grotesque, painful methods such as gassing and heart-sticking. Pet stores are one of the main reasons we do this.

There is quite an argument about this issue in the comments here.