Juliejuliette has it right. Aniston plays a girl who is weak and selfish, without any real character. She first seduces an unhappy and introverted young man well played by Gyllenhaal (who is not as disturbed as some viewers think) who is looking for love and when he falls hopelessly in love with her she then finds him a danger to her marriage and betrays him, getting rid of him and driving him to suicide. She doesn't seem to feel very guilty about what she has done and only minimally sad. She is an opportunist who runs away from responsibility for what she does. At the end she is lying to her husband to preserve a marriage she doesn't like. In all she is the opposite of a good girl, and I suppose the title is meant ironically. The movie does present her and her dubious morality in a realistic way. There is nothing sentimental about it.
Review of The Good Girl
