This was the first example in the modern era of a populace uniting in grief over the death of a celebrity - which is what Diana was in the wider scheme of things.
It's not entirely new - thousands turned out to weep and wail over the death of the silent film star Rudolph Valentino.
It was, and remains, a fascinating phenomenon, that people do appear to feel grief at the death of someone they knew vicariously.
In many ways, it speaks to the same voracious desire to be 'involved' that feeds celebrity culture in general, and specifically the endless appetite for more and more images of the famous, which ironically may have contributed to the crash - since the car was being pursued by the usual hoards of paparazzi.
Why do we need this constant update of images and 'information' about people we don't know? And why are so many people still consumed by the passing of the biggest celebrity of all?
I don't know - but I am sure there is one or more psychologists preparing talks for cruise ships based on that very subject.