Fire
Long before the Time War, he saw a planet burn: a parallel Earth, destroyed by Stahlman's drilling operation. He'd tried to stop them, confident that he would prevail, as he had so often. Misplaced confidence.
Fire filled his dreams upon his return to his own universe. Night after night, he relived his failure in nightmares so vivid he had to leave the TARDIS and walk through UNIT headquarters, or go outside, to reassure himself that he had succeeded in saving this Earth. He became testy, impatient, chafing at both his imprisonment on Earth and his guilt-laced grief.
Time's passing, freedom, and change of companions lessened these wakeful nights, but he had to regenerate for the nightmares to end.
And so he lived, traveled, gained and lost friends, regenerated, saved planets and the universe from destruction. He failed at times, and when he did not wake with his screams echoing in his ears, he thought he'd learned to accept failure and loss.
Until the Time War. When he learned differently.
Now?
There are no screams. No nightmares. And he wishes for both.