Eliot, who herself engaged in what was in Victorian England a highly unorthodox unmarried relationship with George Lewes, critiques the damage conventional marriage can wreak. When the sensitive, passionate and intelligent Dorothea decides to yoke herself, under romantic illusions, to the older Reverend Casaubon, she experiences a crushing disillusion. She thought she would be supporting a great man embarked on a great work of knowledge, throwing her all into him, but instead, she finds herself as companion and nursemaid to a highly conventional, jealous man. She finds after the marriage that he doesn't have the breadth of mind or vision to complete anything great and that she is trapped.
Likewise, Lydgate, full of hopes, dreams and ideals about doing pure medical research that will help mankind, yokes himself to the pretty and very conventional, narrow-minded Rosamond, who wants money, status and pretty things. She doesn't understand or support Lydgate's dreams and he ends up a fashionable London doctor to the rich, making money but at the price of abandoning his youthful dreams.
In both cases, the intelligent, sensitive partner in a marriage is damaged by marriage conventions, Dorothea by the expectation that she will subordinate herself to her husband, no matter how unworthy he is, and Lydgate by the expectation that he will materially support his wife in the style to which she is accustomed, no matter what. Eliot thus critiques an institution that can have a soul-crushing effect on sensitive, intelligent people, preventing them, because of social norms, from achieving their potential.