Comparisons to all the starving children who die every day, etc, often don't take into account effects of perspective and psychology. I think Christopher Reeve is a particularly good person to examine this with, because he's not a random celeb - he had the honor of portraying one of the most recognized cultural icons of the 20th century (an icon that for many people embodies everything noble about the human race), and when he was humbled by a terrible accident, was steadfast and brave under circumstances were being famous and wealthy don't make your problems go away.
So Reeve was a true symbol for a lot of people. If we examined our feelings carefully, we might find that we're sad not because we honestly think his life as just a human being is more valuable than others, but because of more abstract and symbolic connections.
We feel emotions (or alternately, don't) at things like death for a lot more reasons than just the fact of death. It's very particular to the circumstances surrounding individual instances. This seems not widely understood or accepted though. (With irony, particularly among people with nihilistic attitudes who say we universally -shouldn't- care or be affected by things.)