One can surmise that Macbeth trusts the witches instead of his close friend Banquo because they have successfully predicted that he would be named Thane of Cawdor. Given the fact that one of their predictions has already come true, Macbeth has faith that the third prophecy will be fulfilled. Macbeth is also significantly influenced by his own ambition, and the thought of becoming king is remarkably appealing to him.
