My husband and I recently struggled with this. Our 9 yr. old girl, the smartest, kindest GSD became very lame and painful with arthritis. Pain meds worked a little, but our sweet girl had one love in life....to play fetch with us. She lived for that! She did not understand she had to slow down, she wanted to run as a young dog! When we did not play with her, she was sad, thinking she did wrong. When we did play, she would run to get the toy, and then cry when returning it. This broke our hearts! I was researching when to decide to say good bye. This phrase struck me hard..."Dogs like to live a happy, fun, pain free life. When your dog can no longer do this, it is time." She was not happy, she could not play and she was in pain, probably more than we knew. We decided we were keeping her around for us, and made the decision to say good-bye. IT WAS SO HARD, but we know she is waiting for us at the Rainbow bridge with a frisbe in her mouth and she will run to us like a young pup again!