When Macbeth first encounters the witches in Act I, Scene III, he is curious and intrigued by their presence. He asks what they are, for example, and wants to know more about their prophecies. He is also confused as to how these prophecies might come true.
Macbeth meets with the witches again in Act IV, Scene I. Remember that this time, Macbeth deliberately seeks them out. His attitude is very demanding: he insists that they answer his questions about the future. Moreover, he says he does not care what they have to do to provide these answers; he even tells them to summon their masters.
When he has received the answers he seeks, he thanks the witches for their advice ("good caution") and claims that if he had three ears, he would listen with all three.
It is clear, then, that Macbeth's reaction has changed considerably. He has moved from a position of interest to one of dependence. He believes wholeheartedly in what the witches tell him and needs their guidance to rule.