(She wasn't here. She hadn't been here for his admittedly entirely self-serving explanation of the Laws of Time.
He smiles softly.)
There are Laws of Time, Layla. The oldest and most powerful race in the universe wrote them to protect the universe. They swore a policy of non-intervention -- to watch the universe from the sidelines and not to alter history.
History has a set path, more or less. It's not a line, like you'd think, but we'll go with a line for the sake of ease. There are multiple lines; certain events are supposed to happen, whereas others are unimportant. For the most part, it doesn't matter who lives and who dies. (He pauses for a moment; he's never followed the rules. He's always blatantly disregarded the Laws of Time. He's a criminal by the standards of everyone in the universe and yet here he is, explaining the Laws of Time for a second time as if they mean something to him. As if he's the Doctor.)
But, if you just take people from their timelines with utter abandon, something's going to go wrong, isn't it? You could take someone who's meant to do something in the future. Some of our future crimes, through their inaction, could lead to the destruction of worlds and the wiping out of entire peoples.
And then there's parallel worlds. Now that is a mess!