"Good sir, why do you start, and seem to fear / Things that do sound so fair?" What does Macbeth's initial reaction to the witches' prophecy suggest, in act one scene 3?

The witches greet Macbeth as Thane of Glamis, which he knows he is, but also as Thane of Cawdor and King of Scotland, two titles he does not hold. In this quote, Banquo asks Macbeth why he acts startled and seems afraid of the prophecy. Macbeth's reaction would suggest that the witches are expressing his innermost thoughts and desires—the wild ambitions, especially to be king, that he reveals to nobody. He would be startled and afraid because he would wonder how these weird creatures could so precisely know his innermost, buried dreams and wishes. He would also be afraid because of his realization that his ambitions are so potent and alive to him that any encouragement might inspire him to take steps that he would later regret.
Yet for all his fears, Macbeth shows his palpable thirst to know more, saying to the witches, right before they disappear, that they should stay and explain how he could possibly become Thane of Cawdor or King:

Stay, you imperfect speakers, tell me more.
By Sinel’s death I know I am thane of Glamis.
But how of Cawdor? The thane of Cawdor lives,
A prosperous gentleman, and to be king
Stands not within the prospect of belief . . .