Answer:
It's because subconsciously, humans value themselves as more important than animals. Human euthanasia is considered unethical if the person isn't able to talk and communicate that they don't want to suffer. Alternatively, doctors can offer that choice to the family of the person who has to make that decision. My dog was suffering when i was younger. He damaged his organs and was internally bleeding. We took him to the vet and while I hated it, I let the vet kill my dog because he looked terrified. He knew he was dying even if we chose not to end it early. I don't think it's a contradiction because I, personally, would've wanted to die if I was in that state or if I was unable to talk and the only thing keeping me alive was a machine.