My family had this decision to make about our childhood dog a number of years back. Like some others have said, it might be best to decide by looking at quality of life. For our sweet Harry, it was probably best: he was 15, going deaf, losing his teeth, and arthritic. Many of the things he really loved doing just weren't possible anymore. An additional concern was the fact that our parents were getting older and my dad was pretty unhealthy, so there were worries about their having to perhaps carry him outside to pee (he weighed 45 lbs.). I still miss my Harry even 23 years later, but I know it was the right decision for us.
There are a lot of factors to consider, but IMO, the top of the list is quality of life. I can't stand to see any living thing suffer. |