Aileen noticed it first after they had been back from Europe nearly a year.

At this time she was still interested in Sohlberg, but in a harmlessly flirtatious way.
She thought he might be interesting physically, but would he be as delightful as Cowperwood?
Never! When she felt that Cowperwood himself might be changing she pulled herself up at once, and when Antoinette appeared--the carriage incident--Sohlberg lost his, at best, unstable charm.

She began to meditate on what a terrible thing it would be to lose Cowperwood, seeing that she had failed to establish herself socially.

Perhaps that had something to do with his defection.

No doubt it had.