She fails to examine whether Casaubon is good enough for her, instead seeking to make herself good enough for him.
Eliot uses the opening quote of Chapter Two to compare Dorothea to Don Quixote. In that chapter, Dorothea sees Casaubon as similar to John Locke while her sister Celia sees an old, mummified "dried bookworm," boring to hear talk.
Like Don Quixote, Dorothea sees what is not there. To her, Casaubon is an intellectual with much experience of the world. Others see him as unintellectual and a failed scholar. Casaubon shows that he is not studious enough to learn German in order to be up-to-date on his research. Ignoring the insistence of other scholars, Casaubon stubbornly refuses to study modern theories. His stubbornness is a chief flaw that Dorothea fails to see. In their relationship, his way is always supreme. As one who is used to asserting her independence, Dorothea struggles to quiet her opinions. Unfortunately, she envisions Casaubon to be the fatherly, intellectual man of her dreams, incapable of seeing his true nature until they are already married.
Dorothea's desire for a fatherly figure in a husband is a result of her traumatic upbringing. Both of her parents were dead by the time she was twelve. She had no brothers and the only male of significance in her life is her uncle Mr. Brooke, who is nearly sixty. Dorothea, who is 19 at the outset of the novel, has very little interaction with males her age. Therefore, she has little opportunity to gauge the characteristics she finds attractive in her counterparts. Some of her interactions with males have come through reading their writings, through which she has been introduced to Blaise Pascal and Jeremy Taylor. These interactions have likely influenced her desires in male companionship. She approaches a relationship theoretically rather than with a love perspective.
Similarly, Dorothea has not had a mother during her teenage years to teach her the characteristics she should look for in a lover. Neither has she had anyone to help introduce her to other males her age, a duty Mr. Brooke has ignored. Consequently, Dorothea is allowed to develop her own preferences, which prove to be misguided.