Dying is another rational fear.  People are scared to die because it means they will be forgotten and will rot in the ground, sit in an urn, etc.  Death goes hand in hand with the unknown as well  as finality. Once someone dies, they're dead. 



Another reason why the fear of death is so common is that death can happen in so many different ways and so expectantly.  Natural causes aren't the only ways to die, but everyday things can potentially kill you as well.