A 13 year old boy and a 12 year old girl met while both in a place where they were forced to reveal things about themselves most people never reveal to anyone, even their closest friends and family. Despite (or because of) what they had learned about each other, they accepted each other, bonded, became close friends and more.
On the day when the boy was finally leaving that place, he went to say goodbye to this very special girl. When she saw him, her face lit up the room with the most beautiful smile he'd ever seen, genuine joy at seeing him and obvious feelings showing in her eyes, mirrored in his own. It was at this moment that he fell in love with her, his first love, and his heart forevermore belonged to her. But before they could tell each other anything of what they truly felt, they were cruelly interrupted and the brief moment was lost in time. They lost contact with each other in the months that followed, but he never stopped thinking about her. She lived in his heart, his dreams, and that memory of her smile permeated his life.
Though he moved on with his life, had relationships, even married, he never forgot her, never stopped regretting that lost moment, and kept searching for her over the years. He even made clear to anyone he was romantically involved with that his heart truly belonged to her and, if he ever found her and she still felt for him, he would give up everything to go to her. As the years passed, he knew that she'd probably forgotten him, even his name, and didn't expect that she would still have any feelings for him if he did ever find her. Time changes people, life goes on, and she became more and more a dream than reality, but he kept searching.

27 years later, he did find her ... only to find she'd died peacefully in her sleep 11 years earlier.
He would never get the chance to tell her how he felt about her. He would never find out how she truly felt about him, then or now. He would never be able to offer her the heart that had always been hers and discover if she would accept it. All he had of her now was a single letter she'd sent him that he'd kept all these years and the painfully beautiful memory of the last time he ever saw her.
His heart still belongs to her. She only has to claim it.