Author's Note: A couple plot bunnies I've had stuck in my head since Rosie was introduced. I own nothing.
Sherlock's affection for Rosie exceeded all John's expectations of the man that he had known for almost ten years. In all the time he had known Sherlock, the detective had never expressed emotion so openly as he did with Rosie.
Rosie would listen to Sherlock expound case theories sometimes. The little girl adored Sherlock, and it showed in these moments on her expressive face.
One day Sherlock came to a breakthrough in the case he was working on while talking it out to Rosie. John watched the familiar expression come across Sherlock's face, the one that meant all the pieces had just come together.
"Oh, Rosie, you're brilliant!" Sherlock had said, and he had swooped down, picked the giggling girl up, and kissed her on the forehead.
Rosie drifted off to sleep at night to violin music. She became so accustomed to the orchestral lullaby that she refused to go to sleep without it on the nights when Sherlock was out on some case-related errand. He played her his own compositions as well as songs that John had named his favorites.
Rosie found Sherlock's moments of deep thought very amusing. When Sherlock was lost to the world, engrossed in his Mind Palace, Rosie tiptoed up to his armchair, in which he was sitting motionless, hands steepled as usual, and crawl into his lap. The longer it took for Sherlock to notice that she had joined him on the chair, the more Rosie giggled. Eventually, Sherlock would come back to himself, blinking and looking down at the little girl, and Rosie would give a final guffaw and tuck her head under his chin. Sherlock invariably rolled his eyes affectionately and wrapped his arms around her.
John loved watching them in these moments. His daughter and his best friend.