I've always given dogs credit for much more intellegence than science leads us to believe. This has been proven to me time and again after a lifetime of observing their behavior.
This was driven home to me once again recently being close to the death of a dog I was quite close to. At close to fifteen years old, this little Cocker was dying and he knew it. He was suffering grand mal seizures. His CBC (blood count) did not have a single value in the normal range. His body was simply shutting down.
I'm convinced that dogs are aware that their end is near, and often try to get off by themselves, dig themselves under brush or trees, and prepare to die. Are they trying to spare their human companions of witnessing their demise, or just looking for a quiet place to peacefully pass over alone?
This dog exhibited this behavior, and I've seen other dogs of my own do it.
It would be better if we could know exactly when the right time is to end our friends' sufferings, but sometimes we're just to close, and too unmilling to lose a friend, to see clearly.
But, there are also stories of dogs reportedly knowing when it's master is coming home, before a car ever pulls up. Or, a dog that can sense when a seizure is coming upon a family member before it materializes.. Or dogs that can "smell" cancer in the body of someone it's close to.
Too many incidents to chalk it all up to chance!
There are many things in this world that Man does not know or fully understand, and I've been a firm believer for a long time that canine intellegence is one of them...