One eternal question in regards to our existence is about death and its mystery.
That is why we cannot remain indifferent to the sight of a skull. We easily recognize the human features of this symbol, due to the nature of our mind to search for familiar faces.
However, at the same time, we realize that this face does not belong to a living person. Thus, this symbol offers a straightforward reminder for the mortality of human beings and the impermanence of life itself.