On her 18th birthday, Bella Swan wakes up from a dream in which she sees herself as an old woman. She expresses her distaste about growing older than her boyfriend Edward Cullen, a vampire who stopped aging physically at 17. Despite her lack of enthusiasm, Edward's adoptive family throw Bella a birthday party. While unwrapping a gift, Bella gets a paper cut. Edward's brother, Jasper, becomes overwhelmed by the scent of Bella's blood and attempts to kill her. Realizing the danger that he and his family pose to Bella, Edward ends their relationship, and the Cullens leave Forks, Washington.
Edward's departure leaves Bella heartbroken and depressed for months; however, when her father, Charlie, finally decides to send her to live with her mother in Florida, Bella refuses and agrees to spend more time with her friends. After seeing a movie with Jessica, Bella sees a group of men on motorcycles. This reminds her of when Edward previously rescued her from an assault, and she sees his image warning her to stay away. Bella discovers that any thrill-seeking activities she engages in evoke Edward's preserved image. She is also comforted by Jacob Black, a cheerful companion who helps to ease her pain over losing Edward. When Jacob suddenly begins avoiding her, Bella discovers that he, and others of his tribe, are descended from a long line of werewolves, and Jacob has just undergone his first transformation. Bella also learns that the werewolves are an age-old enemy of vampires. Jacob's pack members are on constant alert for Victoria, a vampire seeking to avenge the death of her mate, James, who was killed by Edward after James kidnapped and tried to kill Bella. They rescue Bella from Laurent, when he tries to kill her. With Jacob busy coming to terms with his shape-shifting nature, Bella again finds herself alone, and she returns to seeking thrill-inducing activities.
Bella jumps off a cliff into the ocean believing Edward will come and save her. Alice sees this in her vision and Edward believes Bella committed suicide. Instead Jacob saves her without Edward's knowledge and she and Jacob are about to kiss, but he ceases himself to do so. Edward travels to Italy and attempts to provoke the Volturi (powerful coven who act as vampiric overlords) to kill him by exposing himself as a vampire to humans. Alice, Edward's sister, and Bella rush to Italy to save Edward, and arrive just in time to stop him. Edward explains that he always loved Bella and only left to protect her. However, the Volturi determine that Bella, a human who knows that vampires exist, must either be killed or transformed into a vampire herself. Alice stops them from killing her by sharing her premonition, in which Bella has been transformed, with Aro (Michael Sheen), a Volturi elder who is able to read thoughts through touch. Soon after, they return to Forks and Cullens again settle themselves in Forks. Bella asks Edward to transform her and Cullens vote in favor of that much to Edward and Rosalie's dismay. Later, Jacob reminds Edward, the treaty Cullens and Quileutes made years before that they will not attack each other, as long as the Cullens do not feed on any humans, a necessity for Bella's transformation. The movie concludes with Edward telling Bella that he will change her into a vampire after she marries him.

Question: Why does Jacob start avoiding Bella after the start of their relationship?
Answer: He turns into a werewolf for the first time.