This question addresses the age-old Fate vs. Free-will argument. In the most basic of opinions, Macbeth is 100% open to making his own decisions and acting on them. However, in many interpretations of the play, he lets the influences of others and his own pride get in the way of his rational decision making.
The first person to urge Macbeth to "hurry" the prophecies along is his very own wife. She desperately wants to be queen, now. After some back and forth discussion, she pulls out her arsenal of influence over her husband. First she attacks his manhood ("...to live a coward....") and then asks "...what makes you break this enterprise to me?" (II, vii). This is her own version of "YOU PROMISED!" (II, vii). When he still wavers, she lets him know that she would do it for him, and that if he loved her, he would do it. Of course, he gives in.
Later, after Macbeth seeks the opinions of the witches, he becomes so certain at their tricky answers that he will never be defeated. His pride keeps him from taking proper precautions for war and for caring for those around him. Ultimately, he loses most of his army, his wife, and eventually his life.