Theoretically, people are supposed to put their pets down when they are in pain or lose their body functions, but in reality, who can do it?
I kept my 14 year old little West Highland Terrier alive (by praying over her every time fell on the ground) until she appealed to me, looking pleadingly into my eyes. Since I could never refuse her anything, I mentally gave her my consent, so she lay down and died within five minutes. I felt so devastated I knew I could never have done that on my own.