Most people have to deny a part of their psyche to function day to day. That accountant has to suppress that once childhood dream of inventing a solution to global warming. That real estate agent is constantly turning from the pocket of their brain that imagined becoming the next great sculptor.
For people who have built their entire means of survival by denying a part of themselves, experiencing the death molecule can lead to the realization of a life unlived. For them, accepting that misstep can feel like a threat to their source of food and shelter. So, they fight the trip. Their struggle is the nightmare.