The fear of death lies in the unknown. The uncertainty of when it shall come, how, why and what shall happen afterward if anything. It's unexplored territory to us, though so many people frequent it.
Everyone responds differently to death. Some ignore it, with the hopes that if they do it will somehow *go away.* Others mock it, "Death can't get me, I'm too obsentent." Some are just plain scared of it and live their lives in constant worry. Still others realize that death will come to all men, that though it is an unknown, it is also a certain. It deaths consistency they almost find security.
I don't think that fear of death is a universal concept though. Those who believe, for certain, that there is nothing beyond death fear nothing but dying alone. Those who believe there is something beyond death will usually live accordingly so that they are prepared for the future. Those whose heart is uncertain as to what shall happen after death are usually those who fear. Uncertainty leave room for doubt and as a result a fear as to what might be around the corner.