From the outset, Macbeth has not shown any real fear of the witches. When he and Banquo encounter them the first time, he is his assertive self, demanding of them to:
"Speak, if you can: what are you?"
His response to the witches' prophecy is different from the response of Banquo. Banquo is unaffected but notices that Macbeth "starts" and asks him why he is startled and afraid when the prediction is so favourable. The fact that Macbeth responds in this manner is not because of his fear of the witches but because they have so accurately expressed his ambition to become king: they have verbalised his innermost thoughts and desires, which is confirmed by his later conversation with Lady Macbeth.
After Duncan's assassination and Macbeth's ascension to the throne, he becomes ruthless and bloodthirsty, suspicious and overwrought. He is in a state of continuous anxiety and has now completely given himself over to evil. Therefore he seeks out the witches to provide him with assurances.
On his next encounter, he shows them very little courtesy or respect. The witches' initial warnings and predictions please Macbeth, but he demands to know more and even threatens them with a curse if they do not cater to his request:
"I will be satisfied: deny me this
And an eternal curse fall on you!"
The Macbeth we see at his second encounter with the witches has become the epitome of evil, unlike the man we met earlier, a man whose wife believed was "too full o' the milk of human kindness." The Macbeth we now see has become evil itself, so much so that he dares to threaten the witches with harm without any hesitation or reservation.
Macbeth's second encounter with the witches differs from the first one because he is the one who seeks them out this time. The Macbeth we see now is completely different from the hesitant and skeptical Macbeth we saw at the beginning of the play. The changed Macbeth is a cold-blooded murderer, evil and relentless in his pursuit of attaining invincibility. He depends on the witches and feels he cannot govern without knowing what awaits him.
Having realized that the witches' first prophecy was fulfilled, he puts his trust in them without reserve and demands to know what the future now holds for him:
Answer me to what I ask you . . .
Tell me, thou unknown power.
Macbeth now shows aggression while desiring to obtain the witches' help; his inexorable drive to listen to the witches and believe in what they say demonstrates how he has become blinded in the darkness into which he has plunged himself by killing Duncan and all those others afterwards. This encounter with the witches presents Macbeth's transformation from a doubtful and nervous tough valiant warrior to an evil tyrant, ready to impose his control and power on anyone and anything.
Macbeth is much more confident this time. First of all, he goes after them, so he's more aggressive. Second, he is very demanding. When he finds them, he says, "Even till destruction sicken; answer me / To what I ask you." He is unafraid and wants to know his destiny and what is to come. When they ask him if he'd rather hear the answers from them or their "masters," he answers, "Call 'em; let me see 'em."
He is much more aggressive and ultimately self-destructive. When he threatens them with an "eternal curse" for potentially withholding information, they respond, "show his eyes and grieve his heart." In essence, Macbeth is "bent to know, by the worst means, the worst." He wants any and all information about his future, even if it is negative, the worst, in nature. The equivocal (misleading) nature of the witches sets up his hopes again since their information can be interpreted in alternative ways.