"You are a good man," she said as she touched his arm with her soft hand.
He looked over into her deep eyes full of wonder and such faith. He did not have the heart to tell her the truth.
She never asked why he had rules.
He remembered the day she had leaded against the Tardis's console and asked about why he never went back to change something he regretted so dearly. She said if she could travel around all of space and time, after seeing everything that was worth seeing, she would start to think of all the things she had seen that should have never happed- all the things that brought unnecessary pain.
He said that if she thought there was anything not worth seeing she was wrong- there is beauty and adventure in everything. Plus that would create too many paradoxes for the universe to continue on.
He didn't say that the rules of time travel was something he had created to protect everyone from something much more deadly than Daleks.
When she asked why he never carried any weapons when trouble seemed to find him so easily, he had just smiled and said that they just weren't his thing. That he believed everyone deserved a chance.
He didn't continue. He didn't say how the best way not to fall into old ways was to keep temptation away.
She never asked why he had rules. She assumed they were not his to hold- that time worked on its own and that a man's morals were hardwired into him from the beginning.
He did not tell her she was wrong.
He did not tell her that a good man does not need rules.
Rules are to keep a man from continuing to be a bad one.