Blue Valentine tracks a young couple's relationship from their meeting until the end of a tumultuous and destructive marriage. At the beginning, they are full of excitement at the prospect of a brand new relationship with one another. They both appear extremely happy and good-looking, which will take a strong turn by the end of the film.
The movie flashes back between the beginning of their relationship and their current marriage. Michelle Williams's character, as we find out early on in the film, is having an affair with an old flame. Ryan Gosling's character suspects this relationship but does not confront her. He is stubborn and very controlling of his wife. He senses she is unhappy but he does not believe that he has caused this.
The death of their family dog adds a strain to an already tense relationship, and Mr. Gosling's character thinks he can change whatever is making his wife unhappy. Ms. Williams' character has always seemed mentally unstable that the viewer can pick up by paying close attention. Her affair, while not a major factor in the film, shows how much she has fallen out of love with her husband and longs for another life or at least an end to her marriage. She is disinterested in a physical relationship with her husband. She is moody and more interested in spacing herself from Mr. Gosling's character; concentrated more on her daughter and work life. She is constantly on the move and wants to end her stagnant relationship that is going nowhere.
Unfortunately for Mr. Gosling's character, he never stops trying even when there is no hope. He tries and tries to reconcile but he doesn't realize that he is never going to change and that his ways will never convince his wife to fall back in love with him. The end reflects how there are things that can never mend.