We've had to put down several of our pets. Our first Malamute had liver cancer (she was 12 or so, which is OLD for a Malamute) and it spread to her brain and the she had a huge tumor in her stomach and she started having seizures. She was miserable. Keeping her alive just for ourselves
would have been really selfish. Our old rescue dog, Cocoa, was so old (about 17) and had severe arthritis and other problems. She was bitten by an other dog and then a week later feel down the stairs and we think she broke a vertebrae. She stopped eating, couldn't walk and became incontinent. It was TIME to put her down. Again, keeping her alive would have been selfish on our part.
Our second Malamute got hold of a woodchuck just before it went into hibernation. We have a lot of land and we didn't catch what happened until she got sick. Then we found the gutted 'chuck. She lingered and suffered and spent time at two different veterinary hospitals (and we spent over $2,000) trying to keep her alive, and she died during treatment. I have never seen a dog in so much pain. She had pancreatitis
which is a terribly painful condition, the enzymes from the pancreas release into the blood and the body starts to digest itself. We didn't know it was pancreatitis and that she also had peritonitis until about 2 or 3 days into the illness. I've NEVER seen an animal is no much pain. If we ever have an animal diagnosed with pancreatitis again, we would have them put down in a minute! It's one of the worst things you can watch a living thing go through.
I've done it and I'd do it again, if the animal was suffering. I've seen people keep animals alive long after they are happy and it's torture IMO to force them to stay alive when they are in severe pain that won't get better just so the owner doesn't want to go through grief.
Cats and dogs live less time than humans. They don't always die easily. I'm glad we have the technology to let them go peacefully, rather than have to watch them suffer when their time comes.