On his deathbed, Mr. Dashwood (Tom Wilkinson) tells his son from his first marriage, John (James Fleet), to take care of his second wife (Gemma Jones) and three daughters, Elinor (Emma Thompson), Marianne (Kate Winslet) and Margaret (Emilie Franรงois), since they will inherit nothing. John's greedy and snobbish wife Fanny (Harriet Walter) convinces him to give his half sisters practically nothing financially and they immediately install themselves in the large house, forcing the Dashwood ladies to look for a new home. Fanny invites her brother Edward Ferrars (Hugh Grant) to stay with them. Elinor and Edward soon form a close friendship, but Fanny haughtily tells Mrs. Dashwood that Edward would be disinherited if he married someone of no importance with no money. Mrs. Dashwood understands her meaning completely.
Sir John Middleton (Robert Hardy), a cousin of the widowed Mrs. Dashwood, offers her a small cottage house on his estate, Barton Park in Devonshire. She and her daughters move in, and are frequent guests at Barton Park. The Dashwoods meet the older Colonel Brandon (Alan Rickman), who falls in love with Marianne at first sight. However, Marianne barely acknowledges him although he is carried in the highest regard and shows only kindness towards her and her family.
One afternoon, Marianne takes a walk with Margaret and slips and falls in the rain. She is carried home by the dashing John Willoughby (Greg Wise), with whom Marianne falls in love. They spend a great deal of time together and clearly fall in love to the point that he snips a lock of her hair to carry with him. Upon leaving one evening, Willoughby asks to spend time alone with Marianne the next day, which could only mean he is going to propose. She asks her mother to stay home from church and during the service Margaret wonders if he will kneel down while proposing. When they return, the find Marianne in tears and Willoughby out of sorts, excusing himself to leave unexpectedly and inexplicably for London.
Sir John's mother-in-law, Mrs. Jennings (Elizabeth Spriggs), invites her daughter and son-in-law, Mr. and Mrs. Palmer (Hugh Laurie and Imelda Staunton), to visit. They bring with them the impoverished Lucy Steele (Imogen Stubbs). Lucy confides in Elinor that she and Edward have been engaged secretly for five years, dashing Elinor's hopes of a future with him. Mrs. Jennings offers to take Lucy, Elinor, and Marianne to London. Marianne and Lucy are excited to go since both of their loves are there. Marianne repeatedly reaches out to Willoughby upon their arrival, but receives no response although the talk in town is that they are engaged. When their paths finally cross at a ball. Although Marianne is overwhelmed with happiness to see her love, he greets her coldly. Due to his demeanor, Marianne faints while exiting. The next morning, all of Marianne's letters to him are returned with a note from Willoughby and the lock of her hair that he cut. He apologizes for any misunderstanding that may have caused her to believe that he loved her. Marianne is overwhelmed with grief, but Elinor is incensed since it was clear to her entire family that he loved Marianne greatly. Mrs. Jennings then divulges that Willoughby is now engaged to Miss Grey, the woman he attended the ball with, who has a large inheritance.
Lucy is invited to stay with John and Fanny, as a way for Fanny to avoid inviting the Dashwood sisters to visit them. Lucy, falsely believing that she has a friend in Fanny, confides her clandestine engagement to Edward and is thrown out of the house. Edward's mother demands that he break off the engagement. When he refuses, she arranges to have his fortune transferred to his younger brother, Robert (Richard Lumsden). On hearing this, Colonel Brandon offers Edward the living of the parish on his estate, feeling sympathy for the honorable Edward. He also explains to Elinor why Willoughby left Marianne: he had impregnated Brandon's ward Beth, the illegitimate daughter of his former love Eliza. Willoughby's aunt Lady Allen disinherited him upon the discovery, forcing him to find a wealthy woman to marry. Unfortunately, it all occurred the day that he was set to propose to Marianne. Elinor hopes that knowing he truly loved her is enough to make her sister feel better about losing the man she loved.
On their way home to Devonshire, Elinor and Marianne stop for the night at the country estate of the Palmers, who live close enough to see Willoughby's estate. Marianne cannot resist and walks to see it, becoming caught in a torrential rain. Everyone looks for her frantically and Colonel Brandon finds her just in time. She is sick enough that the doctor recommends that the Palmers remove their child from the house and tells Elinor to prepare herself for her sister's passing. Colonel Brandon, sick with worry over Marianne's health begs for a way to help. Elinor requests that he bring their mother to encourage Marianne to fight through her illness. She arrives just her daughter's fever breaks and she is out of danger. Once they return home, Colonel Brandon and Marianne begin spending time together. During her convalescence, Marianne admits to her sister that she was once foolish to love with her whole heart instead of holding back like Elinor. She also admitted that she knew Willoughby couldn't have all he wanted since he either had to choose love or money.
The Dashwoods soon learn that Miss Steele has become Mrs. Ferrars and assume that she is married to Edward. Later when Edward visits their house, they learn that Miss Steele jilted him in favor of his brother Robert once he had the entire inheritance, releasing Edward from his engagement. Edward proposes to and marries Elinor. Edward becomes a vicar, under the patronage of Colonel Brandon, whom Marianne happily marries. Willoughby is seen forlornly watching their wedding from a distance, and then rides away.

Question: Why did John Willoughby leave Marianne?
Answer: He left because he got Brandon's ward Beth pregnant and his aunt Lady Allen disinherited him which forced him to find a someone wealthy to marry.