I have talked to many people about death over the years as it is a subtheme of my life. These are the most common reasons for the fear of death and/or eternity in modern times from my observations.
1. Religious guilt imposed by others during our developmental years. I have found this to be true even in people who have not been directly exposed to any religion in their families since this religious guilt is mostly ancestral and has wormed its way into the collective consciousness so it is imposed in more subtle ways by some. I personally got the sledgehammer approach of religious guilt as a child, by an ESI, which I rejected since it went against my internal truth.
2. Some people project their current circumstances and self image into eternity so if there is any depression, fear or doubt in the present they can't perceive a death and eternity that is nothing like a human lifetime. Sometimes I remind people that they are projecting their physical form into a non-physical existence that is probably nothing like they are imagining. Imagination funny enough is the key to getting past the fear. Imagining a future you that is different than what you see yourself as right now. Energy can neither be created or destroyed so you will only change form. I believe fear of death at its core is the fear is of the unknown.
3. A fear that there isn't enough time to do everything they want to do. This is a form of projecting into the future too but seems a bit like weak N when this is the cause. Edit: Not always weak N since other issues can make it feel like there is a time crunch I guess.
The idea of hell for some people who fear death is stagnation, repetition, being stuck in a loop. It kind of was for me anyway, just for a little while when I was depressed. We have a survival instinct for a good reason and some people need a healthy dose of fear to reinforce it. I just feel happier now that I no longer have the fear of death looming over me all the time. I am definitely not ready to die though.
I have noticed something interesting is those who no longer fear death and are dying. Their survival instinct basically gives way to something else. It is awe inspiring to witness.