He wasn't able to mention to her what he felt.
Each time he looked in her brown eyes, each time he hugged her, each time she smiled 'that smile'. Each time she was in his presence.
His hearts filled with love and adoration for her.
But his head said he must not say it.
He was a Time Lord. She was human.
It couldn't possibly work.
If it did, they would both left heartbroken in the end.
He must resist.
He just didn't know that by the time that Torchwood, the Cybermen and Daleks occurred, and she was trapped forever, it would be too late.