OK, I know this isn't the primary reason why people do them
but one magical thing about sex, crying and eating
is that they reveal our sea-creature-ness to a very visible level.
Perhaps that's why they bring so much relief.
CF ‘The Naked Ape’ - theory that humans evolved near (and in) the sea during a critical geological period and this explains uprightness, lack of hair etc Written by a woman in the 80s, widely pooh-poohed at the time.