He doesn’t remember. He’s John Smith, an ordinary man with an ordinary job, and a woman who loves him. That’s who he wants to be, a good man with a good life. Why can’t he stay that way?
Martha says he’s the Doctor, an alien who travels through space and time. She says everything he remembers is a lie, that the Doctor is needed, that John has to become him again. He doesn’t want to.
If he becomes the Doctor again, John Smith, with all his hopes and dreams, will be gone. It isn’t fair.
But he has no choice.