Just a little something that I wrote quite a while ago, actually.
Hope you like it!
As he drew his arms around her, Rose knew, somehow that she'd made the right choice. She looked up into the eyes of the Doctor, her Doctor, and she felt at home. In that moment, she knew why she hadn't kissed the proper Doctor on Bad Wolf Bay. He never would have been able to confess his love for her. Even if she was with him, she wouldn't be with him. She would always be wondering how he really felt about her. Her Doctor wasn't like that. Her Doctor could give himself to her fully, he would never hold back. He wouldn't be afraid to tell her that he loved her. She could never forget her proper Doctor, but he lived on through her Doctor. As she felt his lips lightly meet hers, she knew that he would never leave her. She knew that she would never be without her Doctor, and now, he would never be without her.
The Doctor walked around the empty TARDIS. His heart felt like it had been ripped in two. He had found his Rose, his precious Rose, only to lose her again. He knew why he had walked away from her without a goodbye. He had already had to let her go once. And now, to see her standing there, in love with a man who looked like him, thought like him, and had all his memories, but who wasn't him, would have tore him apart. He knew he could never have told her that he loved her. And he knew that would have destroyed her. He knew that, if he kept on travelling with her, one day he would have to lose her again. That would have tore him apart. He knew it would have destroyed him to watch the woman he loved wither and die. He knew that he had to let her go. He was just glad that she could be happy.