Casaubon first noticed Dorothea for her intelligence and assertiveness. However, these very qualities make him unhappy after his marriage. Casaubon isn't the "great soul" that Dorothea wants him to be, and she isn't the docile, submissive woman he wants her to be. Casaubon is an insecure man. His life-long work, the Key to All Mythologies, is impossible to complete. He views the process of beginning to write it with apprehension and anxiety.
Dorothea's early adoration of Casaubon's intellectual pursuits bolstered his self-esteem. She thinks her insistence that he stop accumulating notes and begin writing is encouragement. However, Casaubon interprets it as criticism. Before, she admired his project from afar. As his wife, she wants to become involved.
George Eliot sympathetically represents the disappointment of both Casaubon and Dorothea. She presents human nature as a necessarily contradictory thing. The qualities that Casaubon admired before marriage become a threat after marriage. Casaubon views Dorothea's involvement with his project as intellectual rivalry. Her desire to learn Latin and Greek further increases this feeling. As a woman and a wife, her rivalry with his field of research heightens his self-doubt. An unambitious, appreciative wife would bolster his esteem. However, Dorothea only exacerbates his pre-existing anxieties.
Dorothea's passionate, emotional temperament bewilders Casaubon. She needs an emotional response, but he is too strictly rational. His inability to give her what she needs makes him feel inadequate as a husband. The collective effect of these anxieties doesn't dispose him to react positively to Dorothea's relationship with Will. Dorothea's attempt to become involved in his dealings with Will further increases his self-doubt. He takes it as a tacit criticism of his ability to do his duty towards Will.
Casaubon's heart attack forces him to face his mortality. His embittered response to Lydgate's advice reveals his fear of dependence. He doesn't want to enter a second childhood or a period of extreme infirmity. Dorothea's anxious concern for his health increases his feelings of helplessness. These personal difficulties generally highlight Casaubon's fear that he is slowly losing his masculine pride. He cannot mold his wife into a model of appreciative submission; she threatens to rival him in conventionally masculine scholarship, and he feels inadequate to deal with her emotionally. He feels threatened in his capacity to do his duty toward Will, both by Dorothea's interference and Will's rejection of his financial assistance. He must rely on Dorothea after his heart attack. He is continually described as old, unattractive, and dry, descriptions that emphasize his frailty and lack of virility.
Whether Lydgate likes it or not, his flirtation with Rosamond is public material. Their mutual interest in one another angers Rosamond's previously frustrated suitors. Lydgate's naive disdain for the importance of the web of social relations has only succeeded in making him a very unpopular man. His belief that he can work with Bulstrode and still remain independent of any personal or professional consequences is equally naive. Unlike Bulstrode, he can ill afford unpopularity.