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The intermittent silences in conversations suggested a cause for concern. But they thought it was just a case of them settling. Perhaps it was only him.
Not shared by her.

By and by, these awkward silences did settle. It was no longer disquieting. They seemed better off. An inadvertent end. That was her.
Though she assumed the same for him.

They let things slip into routine. A comfortable routine with the absence of the other. They no longer remember the cause(s). They don't want the effect(s) to ruin their lives.
They carry on.