Think of it this way: If I donate my organs, I am prolonging another person’s life. However, that person will eventually die regardless, and nothing can prevent that. Therefore, why should I make any special effort to prolong the inevitable? A total stranger’s fear of death is not my concern, since it is a universal fear that we all face.
What are the potential consequences of this type of thinking? Two possible avenues have occurred to me. One is that the decision would be easier if the person to receive the organ were to perform some important duty to benefit society. But in reality, most people who receive organs are older retirees, whose most productive years are more-or-less behind them.
The other avenue is that people must be kept alive because human life has inherent value. The philosophical corner this puts me in is arguing that the organs of someone in a coma must also be replaced, because their life is still valuable, even if they are a vegetable.
*- I should mention that I am on record as an organ donor myself, and this thread is intended as a philosophical exercise only.