He watched. Because he always watched, even if he didn't appear to do so. Even if he didn't interfere as often as he would like to. But where he could interfere, he did.
However, in cases where he could not...
Those were the times when the Doctor sorely wished for a temporary loss of his perception of timelines.
Unfortunately, some things are just meant to be, like the 456 or Demon's Run or so many, many others! Fixed points in time they were, events or people that had consequences, oftentimes reaching far, far into the future.
And nobody knew better than a Time Lord that to mess with Time... Is NOT to be done. For the consequences of that were always far more terrible than the cause. Always.
Humans did have the merest speck of an inkling of an idea, though, what with that 'Butterfly Effect' theory of theirs, the Doctor admitted. Even if their application of it was rather crude and wonky.
This was the reason why the Last Time Lord just stood in the deepest, darkest shadows of the trees across a certain house in a non-descript suburban neighbourhood somewhere in America. And in the middle of the night to boot!
Stood there, a silent sentinel, his impressive eyebrows drawn into a rather intimidating glower, watching unhappily as anonymous black-clad people noiselessly carried a small girl off into the night. Nobody but one in the house the wiser as to the circumstances or reasons for that abduction.
The Doctor could not – must not! – do anything to prevent her kidnapping, no matter how much he wished to save another little girl from what he knew to be an ugly fate. No matter how much it galled him that this was neccessary, a future driving force for her older brother to choose a career with the Federal Bureau of Investigation. From there onwards, the young boy would see and learn and experience many things that–No. None of that tonight. 'That' was still some years into the future, the Doctor decided. Not for now. Yet.
He scowled, slipping on his Sonic Sunglasses, and turned around to walk back to his Tardis. Maybe he would try to play a new piece or two on his Yamaha "Black Sparkle" to forget-remember that these things happened for a reason. They always did. Whether it was a good one, though...
The Doctor shrugged. He had just about tired of trying to decide on that, preferring to let History be the judge in most cases. Although he had a feeling that his music would be more blue-sy than usual tonight...