An interesting article in the Guardian points a finger towards selfishness, rather than a growing altruistism in society, being a reason for the growing number of public shows of mourning. I have to agree with many of the points put forward. Many people are simply using the public grief to put a face on a real grief they have for a given event. However, I think that many people simply do it because everyone else seems to be doing it. Looking like a caring individual rather than being one would seem to be more important.
(I hasten to add that I am not one who feels real grief at some random celebrity dying, but neither would I want to walk down the street professing mourning just to fit in.)