Why jump on the bandwagon, when the bandwagon is a hearse? There are self-serving reasons: Evolutionary psychologists argue that the public expression of grief boosts your reputation as a trustworthy member of the community.
Sudden tragic death can inspire emotional rubbernecking in anyone. (How many of us have boasted about near misses--say, driving through an intersection five minutes before a fatal crash?) A national catastrophe such as September 11 brings this behavior out of the woodwork. That fall, people felt compelled to disclose that they had friends of friends of friends in the World Trade Center. New Yorkers morbidly compared notes: How close were you? What did you see? Who did you know? (In this creepy social gambit, the "winner" is the person most directly affected by the attack.) The same calculus was at work in other states or countries, where the comparison was not what you saw firsthand but who you knew in New York City or Washington, D.C.