I'd argue it doesn't only concern death, but anything inherently human. Humans have their own set of brains and organs. If you poke one person with a needle in a group of five, only one person will actually feel it. If a father and a mother lose their child, they will feel a very similar pain, but it's two different instances of the same pain. The "high level" human functions like empathy and language allow some kind of similarity or sharing, but on the "low level" each human individual is very separate and far away from the others.
Ideas like a marriage unifying two people into one (even if it actually happens psychologically), or a group of people needing each other to survive is just high level function which cannot exist unless the low level unit of a person, their brain, circulation, digestion, etc work well enough. Such unions do not manifest, do not exist on the lower levels. This is something that people intuitively felt since forever, and pretty much seems to be true at the moment. (There have been opposite intuitions, that certain connections last beyond death, but that would imply a top-down organization of the world, which seems sketchy these days).