The sound of running water triggers something… what? why?
Pavlovian conditioning. (I would assume.)
Doesn’t that [Pavlovian] imply someone took the time to do the conditioning?
Running water will do it for me, but please accept my assurances that no one took the time to condition me in that manner.
This thread makes me have to pee.
You’ve been peeing your whole life. That pee makes a sound very similar to running water. You’ve been conditioned your whole life.
Ok, on a related note - why do cold temperatures make you feel like urinating?
I was told once (while peeing in a field on a wintery day!) that the cold makes your bladder contract.
(This is probably just an old wive’s tale, and I have zero evidence to back it up. But when a bunch of very tired and very cold soldiers all agree, then it has to be true, right? 
Thias was going to be my guess but I neglected to answer as I don’t want to be known on this board as the expert pee-ologyst. 
why not? You sure have the right username.
When exposed to cold, blood vessels in the extremities constrict and more blood volume is diverted to the core. With increased blood volume in the core urine output increases.
Ha! I did not think of that.
Ok, but why when going to pee and having a difficult time getting started, does merely thinking about cold or running water cause you to pee?
Surely thinking about cold doesn’t affect your blood flow and squeeze your bladder.
Of course it can, because you have associated cold with peeing. That is what conditioned responses are all about. Same as thinking about fresh, hot pizza makes your mouth water, or thinking about your lover naked, well, you know.
I remember, as a very young child, my mother going “Pssssssssss Pssssssssss . . .” to get me to urinate. I assume it was before bedtime or a trip.