The film explores several flashbacks and present timelines to show how Dean and Cindy became involved. Dean is a young high school dropout, working for a moving company in Brooklyn. Cindy is a pre-med student living with her constantly fighting parents and also caring for her grandmother in Pennsylvania. Cindy and Dean meet at Cindy's grandmother's nursing home while Dean is delivering a new resident's furniture and they begin dating afterwards.
Cindy discovers she is pregnant, and tells Dean that the baby is most likely not his, as her ex-boyfriend Bobby didn't use protection. Dean asks Cindy whether or not she wants to keep the baby. At an abortion clinic, Cindy decides at the last moment to cancel the procedure, and on a bus ride home, Dean tells her he doesn't mind if the child is not his, and that he wants to begin a family with her. Before the wedding, Bobby finds out about Dean, and beats him up.
Five years later, the couple lives in rural Pennsylvania with their daughter Frankie. Dean works at painting houses while Cindy is a nurse at a clinic. One evening, Dean insists on taking Cindy out for a romantic getaway at a motel so they can have some time off from their preoccupied lives, much to Cindy's reluctance.
While buying wine in a liquor store, Cindy sees Bobby, who asks Cindy if she has ever cheated on her husband. She hesitates, but eventually says no. In the car, Cindy and Dean get into an argument when she mentions seeing Bobby again. At the motel, they continue fighting during sex. Cindy is called away early in the morning to work at the clinic, and she leaves a note for Dean. At the clinic, Cindy's boss, Dr. Feinberg, talks to her about a position he had offered her, and asks if she would move closer to work, suggesting that they would be able to spend time together on weekends. Visibly upset, Cindy says she previously thought he was offering her the position because she was good at her job.
Angered that Cindy left the motel without waking him, Dean shows up drunk at the clinic, leading to a violent altercation with Dr. Feinberg. Cindy says she wants a divorce after Dr. Feinberg fires her. After leaving the clinic, Dean tries to persuade Cindy to give the marriage another chance, asking if she wants their daughter to grow up in a broken home. Cindy says she does not want Frankie to grow up with parents who are so hateful to each other.
Dean reminds Cindy of their wedding vows, and the two apologize to each other. Dean is seen walking away from the house, with Frankie running after him. Dean tells Frankie to go back to her mom despite Frankie begging him to stay. Dean tricks Frankie by challenging her to a race in an attempt to send her back to Cindy, and he continues walking away while Cindy picks up an upset Frankie, who cries "I love him".

Question: Why did Dean and Cindy argue on the way to the hotel?
Answer: Cindy tells Dean about seeing Bobby
Alternative answer: Because Cindy saw Bobby.