(A/N: Originally I meant for them all to be brief sketches like this, so I could update quickly, but things never go as planned. ^ ^)
F: Fleeting, Flutter
John likes to feel his heart pound. He's loved it ever since he was a kid and he first discovered football, and then rugby, and then boxing. Maybe it's part of why he's crap at relationships: because he doesn't understand those softer moments, when your heart isn't pounding, but sighing or yearning or swooning or breaking. John doesn't sit well with those moments. Give him the rush of blood in his ears, the adrenaline flooding his limbs, the shaking aftermath. Give him a gun and a uniform. And when those things are taken away (except he keeps the gun), give him footraces across London and shots fired in the dark. Sherlock keeps John's heart pounding.
When John first hears Sherlock playing the violin, first looks up to see Sherlock looking out the window and swaying, half-dancing with the bow, John's heart does not pound. There's a sort of flutter instead - something akin to the feeling of jumping out of an airplane, mixed up with the feeling of looking into a beautiful girl's eyes and knowing she does not love him. It is intense but fleeting, and when it is gone John tells himself it was a reaction to the music (even though he's never liked any piece of music written before 1960 in his life), and not a reaction to the way Sherlock looked playing it. He even believes it.
Until it happens again.