Do you know the feeling you get when you start a new job? The excitement, the rush. The longing to learn it all, to excel, to savor mastery and competence, and earn the respect of your peers? Do you also know the feeling of having done the same work, over and over, for ten years, no longer learning anything new, no longer trying new things. Having reached the end of your wits and will, being depleted..
Perhaps the reason we cannot live forever is because we were only meant to experience the dulling of a blade so many times. A knife can only be re-sharpened so many times before the blade disappears completely.
There is one thing that is universal, and that is the spark felt from a new experience. Our reaction to new experiences is primordial, primitive, natural. If one were to live forever, then there would no longer be a spark. Everything would have been felt a few times at least, everything seen, everything tasted. Instead of life being a series of sparks, there would only be a monotonous consciousness as long and drab as it is dull and silent. Calloused against the changing of the colors and the seasons, of the coming and the going of empires, the societies and the supernovas. Regardless of the tragedies and the ironies, of the humor and the savagery, of the bitterness and tenderness of this world, nothing will move it. In silence this life, this consciousness that has begun to live forever, will hum in stillness, losing any need to move and think. It will lose all need to interact, and influence, as there is nothing to gain from being known yet again by those in the universe, because every reaction has been experienced. It will lose all thought, for there is no new formulation of ideas, no new stimuli, and no new assembly of concepts to consider. All thoughts have been thought of, at least 5 trillion times in the past, in every variation. It would lose touch with life, with people, and with the world. Living forever would make one all knowing, because one would become the steward of all that this universe has for humans. And as we are driven by the spark of things that are new, this individual consciousness, living forever, would lose its drive. It would lose the need to know and learn, as it has seen the rise and falls of a nearly infinite number of species. It would become god. Forever, all knowing, and detached from the creation.
You will die, or you will lose your taste for life. One of these two can occur.