Final answer:
Macbeth deceives Banquo about his thoughts on the weird sisters to conceal his ambitious plans and protect himself from suspicion, ensuring Banquo does not become an obstacle.
Step-by-step explanation:
Macbeth lies to Banquo about not thinking of the weird sisters because he is deeply consumed by the prophecies that they have presented to him. Macbeth's claim to Banquo that he doesn't think about the witches is a strategic deception to hide his ambitious thoughts and burgeoning plans. These plans involve the prophecy that Macbeth will become king, which implies the usurpation of the current king, Duncan. Macbeth's lie serves as a way to protect himself from suspicion and to prevent Banquo from becoming an obstacle in his quest for power, as Banquo might be more apt to foil Macbeth's plans if he knew the true depth of Macbeth's contemplations and desires.