The Doctor knows Martha looks at him when she thinks he won't notice -- stealing glances from her side of the console to where he'd be busy setting the coordinates for their next adventure. He knows she likes what she sees, and he likes that she's looking. He seduced her (in hindsight he admits to himself that he did) with his smile and makes her stay with hugs and random acts of affection, testing the limits of her human desires before assuming his former nonchalance. He can always chalk it up to his 'alienness' and she'd be none the wiser. He makes her believe that she's closing the gap between them, but the moment she blinks, he's miles in front of her again. He makes sure that in every round he's got the upper hand on things. He makes sure that she'll never win.
But sometimes he slips.
It's so easy to talk to her, to answer all her questions, even the ones that hurt. He had told her about Gallifrey, of its red grass and orange skies, the planet he loves but had lost to the Time War, by his own hands. The way she could make him reveal so much so effortlessly outright scared him at times. He can't have this, this release, not when he has blood on his hands and he should never forget it. So he was careful not to let her win another round. Whatever game he chose to play, this is not one he could afford to lose.
On and on he went, skirting the thin ice that lay between them, not realizing that sooner or later the ice will thaw and he will fall head over heels. (Deep down he thinks that maybe, just maybe, it won't be such a bad way to go.)
The time came when she wanted out.
Martha stood in front of him with eyes so old and a weight of problems she should have never been made to bear. And he knew (though she would never ever admit), it was his entire fault. She walked a year for him, saw horrors beyond his imagining, and all he could do was say "Thank you". Even then, at the end of this game during which he so obsessively kept score, he couldn't say what he really felt because just one word could cause him to lose everything he's held on to. His pride, his ego, his hearts. Bad habits are hard to break, he would know, he's tried it before.
He lost her, and this game is not worth playing anymore.