Why did Pip think that Miss Havisham was his benefactor?
For Pip, who is summoned to "play" at Satis House very early on in the novel, Miss Havisham seems to be the only person who could possibly be his benefactor for various reasons. Primarily, Pip is made aware of his commonness by Estella, the beautiful girl who lives with Miss Havisham and with whom Pip thinks he's in love. Pip's obsession with Estella continues for the entire novel, and when it comes time for Pip to leave for London so that he can become a gentleman, he assumes Miss Havisham is the one behind it. (Obviously, as Estella took pride in pointing out that Pip was beneath her with regard to social class, and because he will become educated in London, there seems to be an obvious connection.)
Further, Miss Havisham's wealth is no secret to anyone who lives near her. Because Pip is raised with Joe and Mrs. Joe, who have very little money, and because the people with whom they're acquainted also have very little money, Miss Havisham again seems to be the only person Pip knows who is financially capable of funding Pip's education.