I agree that it's a very personal choice. I've mentioned before that when my father was dying, laying there in a hospital bed drowning in his own fluids, and been revived a few times, I told him if he was my dog I would have put him down weeks ago. He laughed and said it wasn't that bad yet. I told him to let me know when it was. A few days later, he said he'd had enough, and they loaded him up with morphine. He woke up once from that and we had a good laugh when he said "I'm not dead yet." in a reference to Monte Python. He passed the next morning.