There are a lot of ambitious characters in Middlemarch. Casaubon wants to discover something fundamental about how all the mythologies in the world are related; Dorothea wants to do something grand to make the world a better place; Lydgate wants to discover some fundamental principle of biology. Does it matter whether these goals are realistic or attainable, or is it just important to have the dream in the first place?
Because of the contingencies of everyday reality in Middlemarch, the only dreams that can come true are the ones that don't have an obvious impact on the wider world; George Eliot thus suggests that the modern world thwarts even the noblest attempts to live an "epic life."
Dorothea's early dreams might not be fulfilled in the way that she had anticipated, yet her life finally had as great of an impact as Saint Theresa's, though she might rest in "an unvisited tomb."