It was the soft rise and fall of her chest, the fan of rich red hair, the complete peace of her in this state that finally pushed him over the edge. He sobbed silently beside her bed, willing him self to stop even as he felt the old wounds reopen. He had lost so much, so many. And now, as if by some God sent gift, she is here. And he knows she will have to leave too, at some point she will have to. She will leave him, or be taken from him, or all other manner of awful things. And he just can not take it anymore. He cannot lose anymore people. He can not lose her. Amelia Pond, with her flaming red hair and her deep hazel eyes and her strange way of thinking. Strange to him at least. He cannot lose her, not now that she is here and with him and so very happy. He can never take that smile from her. He knows he must some day, before she gets hurt by either him or the things he attracts. But his battered and broken hearts cannot conceive of letting her go. Anymore than he could ever conceive of letting go of Rose. He stops himself there. He knows this is a dangerous train of thought. And so he stands, sobs still shaking his slender frame, and leaves her to sleep in peace.
"Morin'" Amy bounces cheerfully into the central room of the TARDIS and he can't help but feel the tug of his lips as he drinks in her infectious smile. Though he hasn't shaken off all the thoughts from last night, he has reached a decision.
He will take her somewhere. Somewhere spectacular and amazing and incredible. And beautiful. Somewhere so beautiful that it will be etched into her memory forever. Even as he hopes, half-heartedly, that she will be able to live life forgetting him. One last trip together and then he will take her home. And will promise her that it will be like he never existed. And he will leave her. Not because he wants to; no, he wants to keep her with him in such a selfish and demanding way that the strength of his feelings shock even him. But because he knows it is the right things to do. For her. And she is what matters. He spent the night watching Rose disappear through the void; seeing Martha walking away; watching the pain on Donna's face as her brain imploded. He will not let this happen again.
And so he paints on a smile. Laughs with enthusiasm. Jumps with joy, literally. For her. For this one last time with her and for her. And the world is beautiful and she is happy. And then she is leaning into him, his arms around her. She is leaning into him and he can smell the coconut scent of her hair and her soft body in warm in his embrace and they are kissing and all he can think is of how he must find the strength to say goodbye.