…why the sound of running water gives you a sudden urge to urinate?
I know it affects women, too, because whenever my wife tells me “Find a place to stop the car, I need to go to the bathroom”, I always tell her “Just think of the ocean.” It always gets a spirited reaction from her.
I’m only speculating here, but my guess is its a hominid survival adaptation, drilled into us by millions of years of evolution. Rather than all the members of a nomadic primate band relieving themselves whenever they wanted, leaving an easily followed scent trail for predators to pick up, they were all triggered into urinating at once, by the sound of a stream, and the urine was carried away and dispersed or diluted. The lost fluid could easily be replaced by all the band, at the same time. It served to coordinate their purely physiological activity.
It’s probably a similar reason why yawning is contagious, or how women living together tend to synchronize their menstrual cycles. But I’m willing to consider alternative suggestions.