Some things you had to earn.

You couldn't just be a Doctor. Not in Martha Jones' world, anyway. You had to take exams, and work. Martha knew how to do it, and she was working for it. She understood that.

You had to earn the right to be his companion. Not anyone could just waltz onto the TARDIS and travel with the Doctor. Martha was lucky enough to have earned that. She didn't know how, but she didn't suppose it really mattered.

And you had to earn his love. But no matter how hard she tried, she'd never managed that.