Martha had entertained hopes at first. Here she was, travelling with an honest-to-God alien, who was undeniably cute, getting whisked from earth’s past, to an alien planet, to the distant future… It was amazing, and so was the Doctor, and who could blame her for being smitten? If he’d felt the same way about her then everything would’ve been perfect.
But he didn’t. The Doctor only saw her as a friend and travelling companion because his hearts still belonged to the girl he’d travelled with before, the mysterious Rose Tyler.
As far as he was concerned, Martha was no substitute.