It starts with the death of companion animals. Not when you are young and comparatively resilient, but later in adult life when you are personally responsible for all of the decisions and costs, the management of a slow and painful decline. The futile delaying actions, the ugly realizations, the fading away, the lost capacity, the indignities and the pain, and all in an individual fully capable of feeling, but who lacks the ability to comprehend what is happening, or to help resist it. One slowly realizes that this is just a practice run for what will happen to everyone you know, later, and then to you. Ultimately it comes to euthanasia, and one sits there looking down at an animal who is a shadow of his or her previous self, second guessing oneself on degree of suffering, degree of spark and verve remaining. It is rarely a clear-cut choice, as in most companion animals the body fails before the mind. When it is clear, and your companion is dying in front of you, you will rush, and later chew it over for a long time afterwards; did you wait too long, could you have done better?
At some point you will ask yourself: why am I trying to maximize this life span? Why am I playing at balancing capacity against suffering? Why have I not just drawn an end to it? Why does it matter if a dog, a cat, another animal exists until tomorrow? Next year the animal will be gone without trace. In ten thousand years, it is most likely that you will be gone without trace. In a billion years, nothing recognizable will remain of the present state of humanity, regardless of whether there is continuation of intelligence or not. The great span of time before and after cares nothing for a dying companion animal. There is no meaning beyond whatever meaning you give to any of this, and there is a very thin line between that and the belief that there is no meaning at all, the belief that there is no point. If the animal you lived with will be gone, what was the point of it all? If you will be gone, why are you so fixated on being alive now, or tomorrow, or some arbitrary length of time from now?
It starts with companion animals, and it gnaws at you. The first of the cats and dogs you live with as the responsible party, the thinking party, the one motivated to find some meaning in it all, arrive and age to death between your twenties and your forties. That is traumatic at the end, but you find it was only practice, because by the end of that span of time, the first of the people closest to you start to die, in accidents and in the earlier manifestations of age-related disease. The death by aging of companion animals teaches you grief and the search for rationales - meaningless or otherwise - and you will go on to apply those lessons. To your parents, to mentors, to all of those a generation older who suddenly crumple with age, withering into a hospital or last years in a nursing facility. You are drawn into the sorry details of the pain and the futile attempts to hold on for the ones closest to you, a responsible party again. You are left thinking: why all this suffering? Why do we do this? What does it matter that we are alive? The span of a billion years ahead looms large, made stark and empty by the absence of those dying now, no matter how bustling it might in fact prove to be.
Grief and exposure to the slow collapse of aging in others: these are toxins. These are forms of damage. They eat at you. They diminish you, diminish your willingness to engage, to be alive, to go on. I think that this burden, as much as the physical degeneration of age, is why near all people are accepting of an end. The tiredness is in the mind, the weight of unwanted experiences of death by aging and what those experiences have come to mean to the individual. Human nature just doesn't work well under this load. It becomes easy to flip the switch in your view of the world: on the one side there is earnest work to end future suffering by building incrementally better medical technology, while on the other side lies some form of agreement with those who say that sadness and suffering can be cured by ending all life upon this world. Oh, you might recoil from it put so bluntly, but if you accept that existence doesn't matter, then the gentle, kind, persuasive ending of all entities who suffer or might suffer lies at the logical end of that road. It is just a matter of how far along you are today in your considerations of euthanasia and pain. This is the fall into nihilism, driven by the urge to flee from suffering, and the conviction that your own assemblies of meaning are weak and empty in the face of the grief that is past, and the grief that you know lies ahead.
Not all of the costs of the present human condition are visible as lines upon the face.