He was trying to vindicate himself.

Just why he should care to do so he did not know, only that all at once it was very necessary that he should appear different in the eyes of this girl from, the other men she had known.
"Will you really ?" she asked, turning to look in his face.

"Will you promise that ?" "Why, certainly I will," he said, a trifle embarrassed that she had taken him at his word.

"Of course I will.

I tell you it's nothing to me.