In act 1, scene 3, Macbeth receives seemingly favorable prophecies from the Three Witches. They tell him that he will become the Thane of Cawdor and later the King of Scotland. Shortly after receiving the prophecies, Ross and Angus inform Macbeth that King Duncan has given him the title of Thane of Cawdor, which confirms one of the prophecies. Immediately after the prophecy is confirmed, Macbeth is astonished. He reveals his thoughts during an aside by saying,
If good, why do I yield to that suggestion
Whose horrid image doth unfix my hair
And make my seated heart knock at my ribs,
Against the use of nature? Present fears
Are less than horrible imaginings
The fact that Macbeth contemplates assassinating King Duncan shortly after being named Thane of Cawdor reveals that he is an ambitious, driven, and power-hungry individual. Since the first prophecy was confirmed, Macbeth believes that he will have the opportunity to achieve the higher, more revered position as King of Scotland. Macbeth recognizes that attaining the throne would give him virtually unlimited power, wealth, and prestige. One cannot blame Macbeth for wanting to attain the crown after the first prophecy is fulfilled.
In act 1, scene 7, Macbeth lists several reasons why he should not assassinate King Duncan in his soliloquy before admitting that his "Vaulting ambition" is his only motivating factor for committing regicide. Essentially, Macbeth's ambition and his desire to attain such a prestigious title are why he is not satisfied with being Thane of Cawdor. It is also important to note that Macbeth is already the Thane of Glamis; his becoming Thane of Cawdor is a lateral move, which is not nearly as enticing as becoming the King of Scotland.
Once Macbeth returns to his castle, his wife manipulates and convinces him to follow through with the assassination. Lady Macbeth's ambition is equal to her husband's. Macbeth successfully usurps the throne after murdering King Duncan in his sleep.