Why does Macbeth believe the witches in Shakespeare's Macbeth?
Arguably, Macbeth believes what the witches tell him because it is what he wants to hear. Deep down, he is an ambitious man and is keen to increase his power and prestige beyond the title of Thane of Glamis.
This idea is supported by what happens in act 1, scene 3 when Macbeth asks the witches how they could know these things about his future. Instead of offering him an explanation, they simply vanish from view. However, their lack of explanation does not dissuade Macbeth from believing what they have told him.
While Banquo is very skeptical about the witches' prophecies and does not rush to accept them, Macbeth could not be more convinced. Why? Because the witches tell him what he already wanted to hear. As a result, the prophecies have validated his own ambitions. It is for this reason that he chooses to believe them.
Macbeth believes the witches because it is an easy excuse for him to foster his "vaulting ambition." Considering that ambition is Macbeth's tragic flaw, Macbeth was not in a situation to further his position more than he had already done before the play begins. Thane of Glamis is a wonderful title, but not as exciting as that of King. It is important to note, however, that it isn't the witches alone who convince Macbeth. To say so would be neglecting one of English Literature's most infamous villains: Lady Macbeth. Macbeth doubts himself (and doubts himself again) before finally being convinced by Lady Macbeth that the witches prophesy should be taken as truth. Lady Macbeth, then, shares a similar tragic flaw with her husband, . . . in fact, perhaps more so.