Since the night of the fire and her open talk with him, the doctor knew "everything," and women are troubled in the presence of a man who knows the details of the past.
The shield behind which they hide in social intercourse is a touch of mystery--or at least a hope of mystery.

The doctor, however, was not like other men; he was more similar to a precocious child and she comforted herself in his obvious talent for silence.

If he had been alert, strong, self-confident, she might have hated him because he knew so much about her; but when she noted the pale, thoughtful face, the vast forehead outbalancing the other features, and the wistful, uncertain eyes, she felt nothing towards him stronger than pity.
It is good for a woman to have something which she may pity, a child, an aged parent, or a house-dog.

It provides, in a way, the background against which she acts; so Kate, when in doubt, turned to the doctor, as on this night.

There was a certain cruelty in it, for when she smiled at him the poor doctor became crimson, and when she talked to him his answers stumbled on his tongue; and when she was silent and merely looked at him that was worst of all, for he became unable to manage knife and fork and would sit crumbling bread and looking frightened.
Then he was apt to draw out his glasses and make a move to place them on his nose, but he always caught and checked himself in time--which added to his embarrassment.
These small maneuvres had not lasted long before the girl became aware that the silent attention of Whistling Dan had passed from her to the doctor--and held steadily upon him.