User blog comment:That One Dino/Why are we Afraid of the Dark?/@comment-9884526-20120326191659

We're not afraid of the dark. We're afraid of what could be hiding in the dark. We can't see what's there, and thus lose one of our primary ways of detecting a threat.

Aloneness means no one to talk to or deal with (humans are very social animals) and death is the ultimate inevitability in life, and we've never found out what happens afterwards.