They say it's death and taxes as the two constants of any man's (Or woman's, I aim to be generic, not sexist) life, but in my case, I would argue Prom Nights and Weddings. It's proven time and time again to be a FASCINATING phenomenon, perhaps one worth a social history.
Discounts for the grooms, the groomsmen and the graduating senior boys (...or girls if you opt to wear a tuxedo I suppose) in any case.