Let us say (but not out loud), that when he smiles, those tired old tunes you make are incited to brim with fire. The baritone of his voice lulling you , allowing everything else to fall away, the doubt and the fear and the nightmares of war that you drag along as incidental music with the accompanying woodwind of the London streets. Let us say that down to the very marrow, the cells, the haemoglobin of your body, there is a reaction of heightened emotion, like hearing a note so pure it is painful, which makes smiling back at him inevitable.
Let us say that even under the sediment of arrogance and distance he has caked over himself, you know his skin replicates exactly that luminescent shade of porcelain flesh like the tight skin of a drum you have always imagined, as though something is illuminating him from the inside. And that if you were to reach out, touch his palm, his slender taut-willow fingers with reverence, conjoin your fingers with his as a harmony of two notes and pitches... well, then you might be able to share in some of that brilliance too.
You want to be the composer of your own score, a one man revolutionary who realises the melody of his own mind in the flesh, plays to his personal tune. You went to war to escape from the sound of everything else, took command, control. You felt the sand in the creases of your skin and the sun slanting off the ground and you were alive with the song of the desert. But one man has never been enough to manage anything alone. You want the highest notes, the perfect pitch and rhythm and maddening tone all embedded in the one man, but you'd have to drag these qualities down to your level. You are not of that calibre, a worn out instrument with scars and damage from when life played too rough, second-hand goods, and you would have to take those perfect notes off their pedestals just to appreciate them properly. You'd spoil them then.
Sherlock is those perfect notes. He is all strings, soaring, majestic, melancholic cellos playing deep and sombre, straining to grasp at stars and bigger things. Then he is the violin, becomes the thrumming trilling notes as though they're the stuff he's made of, a human being composed of searing sound and light. The sheet music he follows plays out a battlefield for you, crippling turns and swerves and tumbling scales. Hearing him play, watching him work, his deductions and brilliance, is like being submerged in music so deep you're almost drowning.
He is beautiful. So it makes sense that you think you'll ruin him.
You are the plodding follow-on, the military drum, the metronome keeping time for the march but nothing special. He would have to curtail himself to lower down to you, would likely choose another flighty strings over the awkward percussion you are. It just makes sense.
(At this point in time you forget that sometimes the beat of the metronome orders and calms an erratic rhythm, that the unpredictable tempo of the strings can inspire a faster beat in the drums. At this point you forget how human he is, not composed of notes and scales on a stave, but capable of the same responses to sound as you. Capable of loving just as much and just as irrationally.)
But if Sherlock were to respond to that advance you have hypothetically made, the small inconsequential nature of touch, a tremolo of a heartbeat, a sonata inviting additional sounds, if he curled his palm around yours and held his notes like you were holding your breath, then nothing else would matter.
You would smile at him, the treble clef at the start of a piece, him readying to follow in your two man orchestra. And oh god, it would be glorious to hear.