What must it have been like, for Twelve, to be totally and wholly accepted by River without pause? When he regenerated, Clara rejected him–this angry old man who appeared in place of her fresh faced and lighthearted friend. It took a phone call from Eleven to convince Clara–Clara who Twelve still considered to be his best friend– to give Twelve a chance and stay with him.
How must it have felt to say “Hello Sweetie” and have River respond to him as she always had? She didn’t question that he may be a new man under that new face, she just carried on as if things were as they had always been. She tells him she hates him (without meaning it), tells him his clothes/hair is stupid, says something salacious and tries to sacrifice her life for his.
His River, who hasn’t recognized him all day long, just accepts him for who he is, without reservation or question.