Then the hot water runs out.
Drops of frigid, icy water spray down on you. Running away isn't an option until the soap is washed off. You shiver and balk at this betrayal as your skin sprouts goosebumps.
"But you've done Polar Bear Club jumps into slightly frosted water. How is this different?"
And there you have it.
The difference between hot water running out, shocking you with unexpected cold versus preparing and willingly jumping into an iced body of water. Consent, preparation, willingness, desire...
Notice the difference?