In Act I, when Macbeth first hears the three witches welcoming him on his arrival: Thane of Glamis, Thane of Cawdor, and king hereafter, he doesn’t immediately believe that there is any truth to their words. But when he’s immediately greeted and given word of his new title by Ross, suddenly the thought of becoming king is sounding more likely every minute. Of course, initially, Macbeth is not viewed as a violent person, a person who could actually make it happen; he’s more likely to bide his time until Duncan dies and gives Macbeth the throne in a more righteous manner (even when this isn’t guaranteed).
On the other hand, Lady Macbeth’s first thoughts when she hears the news are the exact opposite of her husband’s. She thinks that although it would be great for Macbeth himself if he were to be king, it would be even better for herself if she were to be the wife of the king, and receive all the perks that come with the job. She’s very influential in getting Macbeth to want those same things, and the greed eventually causes his unravelling.
And with that, Macbeth kills the king and becomes king himself.
Therefore, when Macbeth sees the three apparitions, he believes that they are true, because so far, his experiences with the witches have given him no reason not to. Macbeth’s greed overpowers his ability to be rational, and if he believes the apparitions are true, he feels secure knowing he can’t be defeated. When it becomes clear that the apparitions are not coming true, his pride gets in the way of acknowledging the truth, and finally, when even his pride gives in to the truth, he continues to hold on to that belief because in the end, everyone needs to have some hope to hold on to, and those 3 apparitions are all that he has left.
In Macbeth by William Shakespeare, the eponymous hero Macbeth is initially skeptical concerning the validity of the three witches' predictions. Despite this, he is intrigued, as they predict his becoming a king. Part of the initial mechanism is roughly the same one that even today causes people to believe in astrology or psychics when they predict good things, that all of us would like some assurance of good things happening in our future.
The point at which Macbeth starts to trust the witches rather than just having his interest piqued is when he first sees their predictions come true. After his meeting with the three sisters, he does become Thane of Cawdor. When this happens, confirming the truth of their prophetic abilities, he also believes their other prediction that he will become king. This prediction is, in a sense, self-fulfilling, as it stirs within Macbeth the ambition that makes him kill Duncan to become king. This act, of course, gives him additional confirmation of their prophetic abilities, leading him to believe their final set of prophecies, which also come to pass.