Why?
She recalled, with more subtle emotions, that he did not look at her now with any of the old light of satisfaction or approval in his eye.
Evidently, along with other things, he was taking her to be getting old and uninteresting.

He saw her wrinkles, perhaps.

She was fading, while he was still preening himself in his elegance and youth.

He was still an interested factor in the merry-makings of the world, while she--but she did not pursue the thought.

She only found the whole situation bitter, and hated him for it thoroughly.
Nothing came of this incident at the time, for the truth is it did not seem conclusive enough to warrant any discussion.