Disclaimer: I don't own Lost.
The Ghost of You
After Charlotte died, Daniel fell apart, literally. For days afterwards, he barely spoke and he walked around in a daze. He couldn't forget she was gone, no matter how hard he tried, but he would still look around for her, hoping that she was still with him.
There were times where he wanted to forget everything about her, but then he felt like kicking himself for even considering that option. Logically, he knew he'd never forget her. He loves her too much for that to happen.
He eventually pulled himself together, but he wasn't the same. He could hear the others talking about him, worrying about his sanity and health. He tried to get back to the way he was, but he knew it was impossible. Knowing Charlotte had changed him, and losing her had changed him more.
He jumped at the first chance he had to leave the Island. There was nothing left for him, not anymore. He didn't want to forget her, but he needed to distance himself from the memories. Everywhere he went on the Island, he could see her. He'd glance into the jungle, and he'd see a flash of red curls. He'd wake up in the middle of the night, and he'd see her sitting on his bed, smiling at him. If he wasn't seeing ghosts of the Charlotte he knew, he'd see her running around the compound as a child, stealing chocolate bars and wrapping the adults around her finger. He needed to leave, or else he would've gone insane.
Leaving the Island didn't help. He saw her more in Ann Arbor than he ever had on the Island. He dreamt about her every night, and he thought he saw her every time he turned his head. Her memory eventually carved out a purpose in him, and he became single minded in his determination. He would do anything he could to save her. If he failed, he just hoped he'd die so he wouldn't have to be without her anymore.
He spent three years perfecting his plan, and nothing could go wrong. He thought of every possible outcome, and he had back up plans for his back up plans. When he came back, everyone thought he was crazy. Maybe he was, but he didn't care anymore. All he cared about was making sure Charlotte wouldn't die.
The moment he stepped off the submarine, he could see Charlotte standing next to him and she hadn't left his side since. She never said anything, but he took comfort in her presence. As soon as he set off for the Hostiles to see his mother, Charlotte had a sad smile on her face, like she knew something he didn't.
The moment he heard the gunshot and felt the pain, he knew why Charlotte hadn't left his side since his return. He knew it was his time to die, and he also instinctively knew that he had failed. As his vision went black, his last thought was that at least he'd be with her again.
