Death is the only certainty about life - one day, we must all die. That is certain.
All the scriptures tell us that we can expect love/harmony/peace/bliss on the other side. So passing over should be a good thing.
Unfortunately, some of the scriptures also warn us about judgement and punishment for sins/infractions. The people who are unsure about how they are going to be judged would surely feel some trepidation. Those who are quite certain that they can expect punishment would be worried, no matter how unconcerned they appear to be. Whatever the case, there is uncertainty about what to expect. The burden of death can be heavy.
Added to that is the visible and experiential pleasantness of this material life. No doubt there are some parts of life that can be painful. But there are also parts that are downright enjoyable. For many people, conscious memories tend to hold the pleasant things and forget the unpleasant ones. Thus, we develop an affinity towards living that we can become quite loathe to give up.
