Dorothea doesn't realize that she's falling in love with Will. She only considers that she's losing the only person she's ever really been able to talk to openly, and all because of Casaubon's unfair suspicions and jealousy.
Dorothea's at dinner at Celia and Sir James' house, and Sir James' mother and Mrs. Cadwallader are there, too.
The subject of second marriages comes up, and Dorothea assures them all that she has no intention of remarrying.
She emphasizes it further to Celia, in private, and tells her sister that she plans on making a Utopian colony instead.