Your logic is flawed, unless you agree with a.ssisted suicide. It is illegal in most states to help put people in pain to sleep. No matter how much constant pain they may have, we cannot put them to sleep. So is it humane to put a dog to sleep because of pain, but not a human being? You have to agree with putting people to sleep who are in a state of incurable pain then. Or agree to being a hypocrite.
but in the end its just an animal. the difference is we would be willing to spend a ton of money to keep people alive, keep them happy, and take care of them. but even then people feel like whoever they are taking care of is draining there own lives. imagine a dog that cant walk and you gotta pick up its s**t and p!ss and feed it and water it. it just aint gonna happen.
and like you said a.ssisted suicide, people in the same scenario want to die themselves. and at least people can be stimulated somehow, video games, tv, or be taken out. what are you gonna do with your dog. let him rot away while he pisses and s**ts on himself everyday.