Definitely saying goodbye. I've raised, trained, and care for many dogs. I've nursed quite a few through sickness and injury. But saying goodbye that last time is always the hardest. I can eventually comfort myself knowing they lived a good life with me, and they're free from the growing discomfort of old age. But there's always a very selfish part of me that wants them to keep going no matter what. To keep living because I don't want to be without them. I often feel very ashamed of that, but it's always there.