Jim and Alicia had two dogs, Bugs and Bunny. They were both 15 years old. Bunny was dying. Bugs was still healthy. They were brother and sister, and had been together since birth. They had never been separated for longer than a couple of hours.
Jim and Alicia weren't sure what to do. The vet had told them that Bunny was in pain, and might continue to suffer for as long as another year. This was unacceptable to both Jim and Alicia. So, putting Bunny to sleep now, instead of letting her suffer, was not an issue. However, what to do about Bugs? The vet said Bugs was still healthy. He could last another two to three years. But how long, they wondered, would he last without his sister?
Bunny had always been Bugs's "big sister"; she was confident and assertive. Bugs was rather timid. He didn't mind when his sister bossed him around; she was also there for him when he was feeling anxious. Bugs followed Bunny everywhere. And whenever Bunny decided to sleep, Bugs always curled up next to her, whether he was sleepy or not.
Jim and Alicia both arrived at the same conclusion without very much discussion. When they took Bunny to the vet so that she could be put to sleep, Bugs went, too.