Here's why this particular prediction holds the greatest interest for Macbeth:
Fulfillment of the First prophecy: The witches' first prophecy comes to pass almost immediately. After the encounter, Macbeth is indeed granted the title of thane of Cawdor by King Duncan. This swift fulfillment shakes Macbeth's doubts about the supernatural power of the witches. It plants seeds of ambition within him, leading him to believe that the second part of the prophecy- he will become king- is also within reach.
Personal Desires: The prophecy aligns perfectly with Macbeth's own secret desire to rule. He starts contemplating how to dethrone Duncan and claim the crown, despite his initial moral reservations. The prediction acts as a catalyst, intensifying his preexisting ambition.
Influence of Lady Macbeth: After learning about the witches' prophecy, Macbeth shares the news with his wife, Lady Macbeth. She actively encourages him to pursue power, dismissing any moral concerns. This reinforcement from his wife, who is just as ambitious as he is, further emboldens Macbeth to take action
Seeds of Doubt and Guilt: Despite his determination to pursue the prophecy's fulfillment, Macbeth grapples with moral quandaries. The witches' prophecy brings forth the darker side of his personality and begins to erode his conscience.
In essence, the witches' prediction of Macbeth becoming Thane of Cawdor and King sparks his ambitions, shakes his moral compass, and sets in motion the dramatic conflict that defines his character throughout the play.