Flame
The light of the candle's flame flickers across Sherlocks face, throwing his cheekbones and chin into sharp relief, and make his eyes change between pal green, light blue, and gray. John sits across the table looking on in amazement, dinner forgotten. Sherlock seems to not notice his friends scrutiny, preferring instead to look out the window, watching for their criminal.
Suddenly Sherlock stands and rushes out with a fleeting "Come along John" thrown over his shoulder. John stands and follows. They chase the criminal trough alleyways and down dark streets. John right on Sherlock's heals as they chase the criminal. And when he throws something back at them, John pulls Sherlock out of the way, thinking of his safety before even his own.
John didn't know he was in love with the worlds only Consulting Detective until that evening. But now he knows. He knows that he loves this man. He knows that there is nothing that could keep them apart. And he knows that he will lay down his life for this man.