The character of Macbeth develops throughout the play. He quickly goes from being a loyal, faithful subject and husband to a vicious, desperate tyrant.
Macbeth and Banquo hear the same prophecies from the witches - but they react in different ways. Why is Macbeth so moved?
The witches speak dangerous thoughts Macbeth is thinking himself. This is probably the most important point. His reaction is as if someone has read his mind.
Macbeth immediately takes one step towards the kingship. Macbeth is excited by the witches' words, but when he receives news he is now Thane of Cawdor, he has proof they know the future and he begins to believe them.
Macbeth is a vulnerable person. The witches choose the right time and place to approach him. He is full of the glory of the recent battle and fresh from fighting and killing.