Answer:
Walk on the living, they don't even mumble. Walk on the dead, they mutter and grumble.
Almost everyone needs it, asks for it, gives it. But almost nobody takes it.
What I am filled, I can point the way. When I am empty. Nothing moves me. I have two skins. One without and one within.
It has no top or bottom, but it can hold flesh, bones, and blood all at the same time.
I am the fountain from which no one can drink. For many I am considered a necessary link. Like gold to all I am sought for, but my continued death brings wealth for all to want more.