I wrote this in response to a thread on Quora.
To answer this question, first I have to point out that someone asking “why” something happens could be asking a number of different questions. What are the mechanisms by which that thing happens? (The mechanistic “why”, also known as “how” instead of “why”.) What is the immediate, or proximal, cause that triggers that thing to happen? What is the big-picture, or distal, cause that leads to that thing happening? What ends does it serve for this thing to happen? (The teleological “why”.)
Many people have replied with answers to this question; answers have appeared to almost all of the different kinds of “why”. I’m pointing this out because I think it’s important to realize that an explanation of a proximal cause and an explanation of a distal cause and an explanation of a biological mechanism are not necessarily contradicting one another even if they are very different stories.
So, yes, we die because we’re going to die anyway (Ian York) and because it’s cheaper (William Pietri). Those are distal causes. Also, we die because our cellular reproduction peters out (Kevin Moore and Matt Langley), that’s a much more proximal cause. There are degrees of proximal-ness; an even more proximal cause of death would be “because our tissues are no longer perfused with oxygen and nutrients because the heart stopped beating” and an even more proximal cause would be “because our tissues kicked in their hypoxic apoptotic mechanisms and that process went far enough that too many tissues were damaged for the system to remain integrated and self-regulating once perfusion was restored”. That latter statement is the closest I know of to an ultimate proximal reason for death; as usual, a very proximate cause also overlaps with a mechanism.
Also, we die because it makes life so much more precious and wonderful (Rick Bruno); this is a teleological “why”.
Finally, note that most of what I wrote above, and most of the answers here, are biological answers about the body. But if you instead think of “you” as the self-consistent stream of information integration we call mind or consciousness, then the proximal cause of death (and, in a sense, the mechanism) is that the system that instantiates the processes necessary to represent that information has stopped functioning. In other words, your mind dies because your body dies, and it has nowhere else to go.
Many people speak these days of a kind of “immortality” based on “uploading” your mind into a computer. I have no idea if this is actually plausible in practice, but it’s very hard to come up with a convincing argument that a mind’s stream-of-consciousness could not IN PRINCIPLE be transferred to a different processing substrate. If this principle ever became a reality, it would enormously change the way we think about death. In that world, death would be like losing a file on your computer that wasn’t backed up. Losing a file that IS backed up is no big deal; likewise, in that world, the death of one physical body, or the destruction of one computer, would only constitute death of the mind(s) it hosted if their information wasn’t also present somewhere else.
As to what that would actually feel like for the mind in question… I have not the slightest clue, and neither does anyone else!