For one thing he was subject to illogical moods--apparently not caring whether she noticed them or not.

For another, he permitted himself the liberty of long and unreasonable silences whenever he pleased.

This she had accepted unquestioningly in the early days when she was a little in awe of him, when the discrepancy of their ages and experiences had not been dissipated by her first presumptuous laughter at his expense.
Now it puzzled her, appearing as a specific trait differentiating him from Man in the abstract.
He had another trick, too, of retiring within himself, even when smiling at her sallies or banteringly evading her challenge to a duel of wits.
At such times he no longer looked very young; she had noticed that more than once.

He looked old, and ill-tempered.
Perhaps some sorrow--the actuality being vague in her mind; perhaps some hidden suffering--but she learned that he had never been wounded in battle and had never even had measles.
The sudden sullen pallor, the capricious fits of silent reserve, the smiling aloofness, she never attributed to the real source.

How could she?
The Incomprehensible Thing was a Finality accomplished according to law.