I'm assuming you're referring to Act I scene 3 lines 51 and 52 in Shakespeare's Macbeth when you ask about Macbeth standing "amazed" at the witches' predictions. The thing is, none of the texts of the play actually say "amazed." You may have read the lines and put them into your own words and thought you were using a synonym, but "amazed" probably isn't an accurate choice. If you read the enotes' etext of the play, which gives you the original words of the play as well as a modern rendering of those words you will find the words "start" as Shakespeare wrote it and "startled" as it's put into contemporary words. The two versions follow:
Good sir, why do you start, and seem to fear
Things that do sound so fair?
Good sir, why are you startled, and seem afraid of
Things that sound so beautiful?
Banquo, who speaks these lines, is reacting to Macbeth's reaction to the two predictions. Macbeth is startled, then, when the witches title him Cawdor (because as he says later "the Thane of Cawdor lives") and when the witches predict he "shalt be king."
Different interpretations are possible here. The obvious may be correct. Macbeth may simply be startled by the obvious: three creatures who, according to Banquo, are withered and wild, appear to be on the earth but not of it, and appear to be women but have beards give him title to a castle that is owned by another Thane and predict he will be king someday. Who wouldn't be startled?
But there could be another explanation.
Later on, after Macbeth has, indeed, been pronounced Thane of Cawdor, he ponders the situation in an aside that no one else on stage can hear and is trying to decide if the two predictions are evil or good, meant to help him or hurt him. When referring to the prediction that he will be king, he wonders out loud that if the prediction is good why does it make him think of "that suggestion/Whose horrid image doth unfix my hair/And make my seated heart knock at my ribs, Against the use of nature" (lines 134-38)? In other words, if the prediction is good then:
...why do I find my thoughts drifting to murder--an image so horrifying that it makes my hair stand on end and my heart pound inside my chest?
Macbeth is already thinking about what it will take for him to be king, and what it will take, he thinks, is killing Duncan, the current king. It is not much of a logical stretch, then, to assume Macbeth has long been wishing to be king and also considering what it would take for him to be king. Notice that the witches do not predict that Macbeth will be king immediately. They predict "hereafter," someday. Macbeth takes that thought and immediately the idea of killing Duncan so he can be king enters his mind. His ambition, as we see throughout the play, is so powerful that it would actually be illogical to think that he hasn't thought about being king and killing Duncan long before the witches' predictions.
If this is accurate, then what startles Macbeth may well be the thought of murdering Duncan. In Macbeth's mind, his being king may be synonymous with killing Duncan. Thus, he is startled by the prediction that he will be king because it means he will indeed go ahead and assassinate Duncan. He'll actually go through with what he has been thinking about doing. And that would startle anyone.