Together to the very end

I believe it depends on the amount of pain a dog is suffering due to it's medical condition. Unlike humans, animals do not understand why they are constantly in pain. If the condition is curable or it can be relieved with medication then of course keep the dog alive. If I had a dog with an uncurable condition that condemns it to suffering for the rest of its natural life, I would rather see it put gently to rest and end its suffering. I think that would be the kindest thing I could do for a suffering animal.