Love
Many novels' major themes include love. At Fault is centered around David's love for Therese. Although she rejected him a couple of times before acquiescing, he insisted on showing her the amount of love he has for her. David openly expresses his feelings for Therese even though she never does the same; in fact, she does the opposite. Yet, his persistence eventually lead to Therese's weakness exposure and she admitted her love and agreed to marry him. Also, David's deep love for Therese made him follow her advice and marry Fanny again after he divorced her.
Another love story that was shown in the novel was Gregoire and Melicent's love. Gregoire loved Melicent so much and expressed that to her in every way possible. The only response he got from her was that she agreed to go out with him to the bayou, but she admitted to Therese that she doesn't like him. Gregoire's extreme devastation when he knows that Melicent is leaving the South is evident, yet Melicent never tells him goodbye. However, Melicent realizes how much she loved Gregoire when he dies and she bereaves.
Advice
The most important advice given and the one which changed lives and the course of the story was the one which Therese gave to David. She advised David to marry Fanny again instead of marrying her because her religion doesn't accept this marriage. Although Therese thought she was doing the right thing, she merely thought of herself. She didn't thoroughly think of the consequences that could arise from making two people who don't love each other marry each other, especially when they were divorced. Also, she underestimated David's love for her, for he married Fanny to please Therese only. Unfortunately, her advice was the worst she had made (she was known as the priest for how much she helped people and gave them advice), as both David and Fanny grew more uncomfortable in their lives. What's more, David had to meet and talk to Therese secretly because he didn't want Fanny to know about his feelings for Therese, but it was too obvious, and Fanny confronted David and ended up drowning and dying after living a very bad life.