Firstly, by fearing death, a person actually fears the unknown. No one knows for sure what will happen to them once they die. Thus, this fear of the unknown, that always accompanies man, makes him fear death and dark alike.
Secondly, the thought that we will have to answer our deeds in the hereafter, frightens us. This is because there are not many persons who are satisfied that they haven't harmed a man by either hand or tongue and have done good deeds throughout their lives. We all speak white lies shamelessly, and do many awful things, some unintentional, while the others we overlook by thinking them worthless. These thoughts come to us as soon as we think of death.
Thirdly, we are too much in love with the world and its superficial beauties. We can't even think of leaving it and its charms. The thought of leaving the world forever itself is dreadful.
These were the main reasons for people to fear death. I came to these conclusions after a thorough study of human nature, which, can never be fully understood.