Thoughts on the severely retarded

When children are born with severe developmental disabilities (like the kind where the kid can't feel any pain and chews of it's own limbs), why isn't euthanasia accepted? It seems cruel to let them survive. We wouldn't allow a dog in a lot of pain or who was suffering live. We would "put it down." Isn't it a violation of human rights (or at least really mean and cruel) to keep some child alive who just hurts all the time, who can't have friends, who doesn't understand, and will probably die young but after years of misery?