I think it was because she thought he was so human that he'd forget everything she 発言しました to him when he turned back. Also, she 発言しました she loved the Doctor to make John Smith think The Doctor wasn't all that lonely, as he was loved.
I think that she thought he wouldn't remember または she was trying to get her feelings across to someone who might remember what she 発言しました so John Smith was her best chance as it was still technically the Doctor.
