Ah, marriage – wedding bells and then happily ever after, right? Close the book, because marriage is the end of the romance? Maybe in Hollywood romantic comedies or Jane Austen novels, but not in life, and Eliot was all about depicting real life. She knew better than most that marriage wasn't the end of the story (her live-in lover, George Henry Lewes, was trapped in an unhappy marriage but unable to divorce). The question of what goes on after the wedding bells, behind closed doors, was really important to Eliot. There are a lot of marriages in Middlemarch, so we get to see a wide variety of samples of what marriage can be like. One of the recurring ideas, though, is that it's hard to understand what really goes on in a marriage (or any relationship) from the outside.
Dorothea's self-abnegation in her marriage to Casaubon turns out to be just as bad as Rosamond's selfish disregard for Lydgate's feelings.
Marriage in Middlemarch is a beginning, not an ending, and Eliot takes great pains to show that there are choices that allow for new beginnings even after the marriage takes place.