The curls. John had a 'thing' for them. Ruly or out of control; gelled or au naturel...he did not care. Luckily, Sherlock had a 'thing' for the fact that John had a 'thing' for his hair; though at times he groused about the length and would have preferred to keep the locks short, he had to admit...
Some days, especially after a chase, on a windy, rainy night, his curls would be a disaster, leaves, dirt, gravel, gum ("How?" "You don't want to know, John"); no one else could unravel the mess as gently or lovingly as John. John would sit on the couch, Sherlock on the floor between his legs, and John would use crap telly to distract his love as he would spend an evening getting to know each and every curl.
"Take your ti-oh Johnnnnnnn-"
As much as John enjoyed this time, and the usual brilliant lovemaking that followed, he would admit, if anyone asked (no one ever did), he much preferred what he called 'morning hair." On the rare days when Sherlock actually slept and actually slept later than John, John would simply watch the morning sun tease the auburn highlights, and wait until those opalescent eyes would flutter open, a smile would slowly grace those miraculous lips...
"You're doing the thing."
"You make me feel-"
He would shake his head, and John would put his hands in the curls, and tenderly pull until Sherlock bent down to bestow a single, sweet kiss on John's lips.
John would brush the waves out of his eyes and sigh.
"I'm the luckiest man in the world."
"No. I am, John."
"In a bit."