Jane Eyre

The film begins with Jane Eyre running away from Thornfield Hall in the middle of the night and finding herself alone on the moors, in the pouring rain. She manages to reach the doorstep of Moor House, the home of Mr. St. John Rivers, a clergyman, and his two sisters. They take Jane in, saving her life.
There follows a flashback, to the 10-year-old Jane Eyre, an orphan, living with her maternal uncle's family, the Reeds, at Gateshead Hall. Janeâs aunt, Sarah Reed, doesn't like Jane and is very cruel to her; Mrs. Reed's three children are also abusive towards her. One day, Jane is locked in the Red Room, where her uncle died, and which Jane believes is haunted. She knocks herself unconscious on the door, after a huge puff of smoke comes down the chimney.
Jane's aunt sends her to Lowood School for Girls, which is run by a cruel clergyman, Mr. Brocklehurst. Mrs. Reed tells him that Jane is a deceitful child and is not to be trusted. Jane tells her aunt how much she hates her and that she is a hard-hearted woman. At Lowood, while another pupil, Helen Burns, is being beaten, Jane accidentally drops her slate. Mr. Brocklehurst brands her a liar and makes her stand on a chair all day. Jane and Helen become close friends, but Helen later dies of consumption (tuberculosis).
Eight years later, Jane leaves Lowood and takes up a post with Alice Fairfax of Thornfield Hall. She will be a governess to AdÃ¨le Varens, a young French orphan girl. When she first arrives at Thornfield, a gloomy, isolated mansion, Jane mistakes Mrs. Fairfax for her employer, but she finds out that she is only the housekeeper for her absent master. While Jane is walking into town to post a letter, a horse passes her and throws its rider. Jane helps the gentleman to his horse. Later, back at the mansion, she learns that the horse rider is Edward Rochester, master of the house. He jokingly tells her that she must have bewitched his horse to make him fall. They gradually fall in love with one another.
One night, Jane is awoken by a strange noise at her door, only to find that Mr. Rochester's room is on fire, which the two of them manage to extinguish. He thanks her for saving his life and holds her hand affectionately. The next day, Rochester leaves Thornfield to visit Lady Blanche Ingram, his future wife; he brings her back to Thornfield with him a few weeks later. When a man named Richard Mason of Spanish Town, Jamaica, shows up, Jane can see that Rochester is disturbed. That night, a scream awakens everyone. Rochester assures his guests it is just a servant's reaction to a nightmare, but after they go back to their rooms, he secretly has Jane tend to a bleeding Mason while he fetches a doctor. Rochester has the doctor take Mason away.
Jane receives a letter from her old nurse, Bessie. Jane's cousin, John Reed, has committed suicide, the news of which has so shocked his mother, Sarah Reed, that it has brought on a stroke. Apparently, Mrs. Reed has been asking to see Jane. Jane returns to Gateshead, where her dying aunt shows her a letter from Jane's paternal uncle, John Eyre, asking for her to go to live with him in Madeira. He wants to adopt Jane and bequeath her at his death. Jane notices that the letter was dated three years ago. Mrs. Reed admits to telling her uncle that Jane had died of typhus at Lowood School. She tells Jane that she (Mrs. Reed) has been cursed by her. Jane forgives her aunt and returns to Thornfield, having begun a correspondence with John Eyre.
Jane informs Rochester that she must leave Thornfield due to his impending marriage to Blanche Ingram. However, Rochester suddenly proclaims his love for Jane and proposes to her; they kiss passionately. During the wedding ceremony, Mr. Mason appears, along with a lawyer, declaring that Mr. Rochester cannot marry Jane, because he is still married to Mr. Mason's sister, Bertha; he adds that his sister is still living at Thornfield Hall. Mr. Rochester admits this is true and takes Jane to meet his wife, calling her his own demon; they find her locked away in a room at Thornfield. Rochester tells Jane that his father wanted him to marry Bertha for her money. Once they were married, he discovered that she was rapidly descending into madness and was forced to lock her away in Thornfield; she was the one responsible for the strange happenings in the house. Refusing to go against her principles, and despite her love for Rochester, Jane leaves Thornfield in the middle of the night.
After Jane regains her health, St. John finds her a teaching position at a nearby charity school. One night, she hears knocking at her door and imagines it to be Rochester, but it turns out to be St. John at the door informing her that her uncle, John Eyre, has died, leaving her all his property and that she is rich, to the tune of Â£20,000 (equivalent to Â£1,600,000 in 2015). Jane offers to share the money with St. John and his sisters, suggesting that they live together at Moor house; they agree to the offer. St. John asks Jane to marry him and go with him to India. Jane agrees to go to India with him, but rejects the marriage proposal, suggesting that they travel as brother and sister, as that's how she sees their relationship. On the moor, Jane suddenly hears Rochester's voice calling her name.
Jane returns to Thornfield, only to find the house a blackened ruin. She learns from Mrs. Fairfax that Rochester's wife set the house on fire and died, jumping from the roof. Jane finds Rochester, but in the rescue attempt he has lost his eyesight. Jane reunites with him and they embrace.

Question: Why must Jane leave thornfield?
Answer: His impending marriage to Blanche Ingram