It was the shock that led to it. It might never have happened but for that. Or so John wanted to believe. How else could he have ended up stroking his flatmate's head? His flatmate's newly shorn head.
Gone were the lush curls that fell in such attractive disarray about Sherlock's face. Gone was the hair that John had (secretly) always wanted to touch. In its place was a severe buzz cut that left Sherlock's neck and high forehead obscenely bare. Ears too. How had he never really noticed how long Sherlock's face was? Attached earlobes, he thought, rather randomly. And shaggy eyebrows.
Sherlock tried to be all bluster about it. "Had to be done. That compound just wouldn't budge, not even with turpentine. It's just hair." But it was clear to John that Sherlock was unhappy with the enforced crop.
John reached out, just meaning to pat Sherlock on the shoulder to say that it was fine, that no one would laugh (not within John's hearing), that Sherlock looked fine (good, actually—could never really look bad).
What actually occurred, was that John touched Sherlock's head and instead of offering supportive platitudes like a good mate, he said, "Oooo, it's so soft!" And then Sherlock put his fingers in John's hair…
And after that, there really was no turning back.