Since they are so common I would guess that it is more a physiological response. Many medications cause vivid dreams so it wouldn't be surprising that something that commonly happens near death has a similar effect. It could be that even the people who don't report it happening are often just not able to remember it happening. I agree there is a difference in how willing different people greet death and that it has some effects but I would guess it more likely relates to how likely someone is to get into the situation where the dreaming happens. I think there is more acceptence of older people accepting death; personally I would say there is danger in overemphasising either willing death or extreme avoidance of death.

One thing that has interested me is the practice of some Buddhist monks of predicting (potentially even to some extent causing) their own death. Long term meditation can increase voluntary control of the autonomic system and I'm guessing something similar happens in willing vs. unwilling death; willingness to die may signal to the autonomic system not to make some types of corrections that could potentially be made.