"Why are you alone?"
It is a question she has been expecting, but even still, Hawke pauses a moment before answering.
She respects the Inquisitor. She thinks they might even have been friends, in a different world. A world where the Kirkwall chantry was not a pile of ash and debris. A world untroubled by an ancient nightmare her blood had unchained.
She would like to be honest with her, but Hawke has always had an uncomfortable relationship with the truth. They are like quarrelsome siblings who can only stand each other from a considerable distance (no, wait, she has one of those). Maybe it is more like a half-estranged uncle who always comes round when it's least convenient (come to think of it, she has one of those too).
Either way, she is not ready to open the door to this particular truth. She is still pretending not to be home, still keeping a wary distance.
So she settles for an oversimplification. It is not an outright lie. Fenris would have gotten himself killed trying to protect her, if she had not taken precautions. It has taken her almost fifteen years and cost her more than she can bear to recall, but she has finally learned to be careful with the lives of those she loves.
But that is not the only reason she left him.
Maybe the easiest explanation is simply this:
Some griefs are not lessened by the passing of time.
Some wounds even love cannot heal.