The overall mood of Middlemarch is a somber one—characters like Dorothea and Lydgate have high hopes for their lives and repeatedly struggle to realize them. Dorothea ends up disappointed in her first marriage to Casaubon, and then, to a lesser extent, in her second marriage to Will (in that she has to let go of her ambitions in order to become a mother). Lydgate, for his part, struggles with falling into debt and then, after ending up implicated in a scandal, must leave the town of Middlemarch and give up his innovative work at the New Hospital.
That said, there are moments of joy and contentment in the novel as well, like when Dorothea and Will finally decide to be together despite Casaubon’s attempts to keep them apart. The final words of the novel effectively capture the somber mood that also contains some hope:
[Dorothea's] finely touched spirit had still its fine issues, though they were not widely visible. Her full nature, like that river of which Cyrus broke the strength, spent itself in channels which had no great name on the earth. But the effect of her being on those around her was incalculably diffusive: for the growing good of the world is partly dependent on unhistoric acts; and that things are not so ill with you and me as they might have been, is half owing to the number who lived faithfully a hidden life, and rest in unvisited tombs.
In this poetic passage, Eliot is saying that, even though Dorothea did not realize her true potential, she was still able to help others—her “unhistoric acts” in her “hidden life” were “incalculably diffusive” and part of the “growing good.” Lydgate’s story ends in a similar way—though he was not able to achieve major medical reform, he helped countless patients to heal and have better lives.