Lesson #3: Only Rose can call him The Doctor

That's news. To other people he suggests John or Mr. Smith. At first Rose thought it was because of the idea of being compared to the "real" Doctor. She thought he did not want this and he was really different from him.

When she asked him, his response made a lot more sense. He said it was a great opportunity, a new life he wanted with Rose and being 'the Doctor' was left behind. Despite he is one himself, the Doctor is now a memory he only likes to remember with Rose.

Well, Rose was much happier with the exclusivity and she knows very well the best time to call him "The Doctor".

The Doctor, the sexiest nickname he could have.