Sherlock Preference

Sherlock:
When he started to miss your company and the butterflies he felt when someone mentioned your name.
"How's (y/n)?" Mycroft asked with a smirk.
"Good." Sherlock answered not wanting Mycroft to look further into it. But he could feel the way he lit up by the sound of your name. That's when he knew.

John:
He realized it when he was sick and you took care of him.
He knew at that moment that he was lucky to have you and that he never wanted to lose you.
"Are you feeling better?" You asked with a worried smile.
"Now that you're here."

Moriarty:
He knew when he realized that you were the only person he didn't have the urge to kill.
You cared about him and he couldn't get enough of you.
"Want me to leave?" You asked when you saw him working.
"No." He said and pulled you to his lap, "I need you right here."

Greg:
He knew when he got jealous of Sherlock. You surprised Greg at the office and got to meet Sherlock. He was scared that Sherlock would impress you and that you'd see that he was a better police than Greg was. But when you placed your arm around him and smiled, he just knew that you weren't going anywhere.

Mycroft:

He knew he loved you when he got a threat about someone hurting you. He couldn't bare the thought of losing you or you being hurt. He did everything in his power to keep you safe and he succeeded. But that's also when he realized to not to take you for granted and to appreciate the sacrifices you've made for him.