[Voronwe]

I wrote a long in-depth post on precisely this subject a while back. I think it is currently lurking somewhere in the Silmarillion Canon Forum.

In short (since repeating the entire post would take forever) I think that the original nature of man (in Tolkien's world) was to live for many many years in Arda and then die out of choice, with no compulsion whatsoever involved. When Men were corrupted by Morgoth, and began to fear death, Eru made death unavoidable so as to save men from Morgoth's dominion. In this sense death, though seeming like a punishment for the Fall, was actually a gift.

Of course this is only my oppinion. However, letter 212 (which I had not read when I wrote the original post) does seem to support it, along with the Athrabeth of course.