Sherlock didn't smile very often. In fact, before John came along, he barely smiled at all. He put one a fake one when he was trying to extort information, but rarely did he ever actually smile.
Then John arrived.
He gave a cursory smile as he entered the room, having already deduced the man's entire life. He smiled when John asked how he knew. He smiled when John asked where they were meeting.
It was the most smiling he'd ever done in less than five minutes.
John came along to the crime scene the next day. John's praise confused him, but still, he smiled. There was something about John that simply made Sherlock happy. They went to Angelo's. Sherlock had never been gay, but Angelo saw the looseness in his shoulders, the lack of tension in his face, the easy way he stood and sat around John- already a part of his life. Angelo joked with them and John denied it. Already, after having knowing each other for only one day, it hurt Sherlock.
That night was also the first night Sherlock properly laughed. They came home, both high on adrenaline and laughed like two fools in the hallway. But they were then interrupted by the drugs bust and the cabbie.
John killed for Sherlock that night. Many people say they would kill for someone, but John actually did. Sherlock found out and that made him smile again. He almost laughed when they were leaving the crime scene and later that night at the chinese restaurant, he laughed with John until midnight.
Sherlock did not generally allow John to see his smiles. He kept them hidden, slipping occasionally, but keeping the mask in place. John never knew just how much Sherlock loved him. John never saw Sherlock smile at his jokes. He never saw Sherlock smile when he teased Mycroft. When John was overprotective, he never saw the fleeting grin cross Sherlock's face.
John didn't know the extent of Sherlock's care. He didn't know how relaxed Sherlock was around him compared to how he was before. John never knew that Sherlock never laughed or smiled or joked before he came along. He never knew that he made Sherlock so much more bearable for everyone at the Yard.
John thought Sherlock didn't love him. He thought the relationship was one sided- that Sherlock only kept him around because he fed him and payed half the rent. If only John knew. If only John saw the smiles saved just for him. If only he saw the convoluted ways Sherlock tried showing he cared. Maybe then he wouldn't have felt so much pain at the fall.
When he saw Sherlock standing atop St. Bart's, his heart died a little. When Sherlock told him he was a fake, his heart died a little. When Sherlock said it was all a lie- that everything John ever believed and everything he built his life around for the past two years was all a lie, his heart died a little. When Sherlock jumped and when John saw his body broken and bleeding on the concrete like a slab of meat, he died. He was just as dead as Sherlock.
John spent the days and weeks and months after the fall thinking about why Sherlock would do that to him. He came to conclusion that Sherlock simply didn't care. Sherlock didn't love John enough to spare him any pain- in fact, Sherlock made him watch as he took his own life. And John cried.
No one knew the extent of Sherlock's love for John, and that was his biggest downfall.