This is a quote from the book "Chicken Soup for the Pet Lover's Soul"

The little boy seemed to accept Belker’s transition without any difficulty or confusion. We sat together for a while after Belker’s death, wondering aloud about the sad fact that animal lives are shorter than human lives. Shane, who had been listening quietly, piped up, “I know why.” Startled, we all turned to him. What came out of his mouth next stunned me. I’d never heard a more comforting explanation. He said, “People are born so that they can learn how to live a good life – like loving everybody all the time and being nice, right?” The six-year-old continued, “Well, dogs already know how to do that, so they don’t have to stay as long.”

Having to put three pets to sleep due to illness, this quote always makes me smile and tear up at the same time. It really is a tough decision, but it's about doing what's right for the animal, not yourself.