In Act III, scene i of Romeo and Juliet,

motivates Mercutio to draw his sword on Tybalt when Romeo will not.

In Act III, scene i of Romeo and Juliet, Mercutio is motivated to draw his sword on Tybalt when Romeo refuses to fight for several reasons.

Firstly, Mercutio is a hot-headed and impulsive character who enjoys engaging in sword fights and verbal sparring. He has a strong sense of honor and believes in defending one's name and reputation. When Tybalt insults Romeo by calling him a "villain," Mercutio perceives it as a direct attack on his friend's honor, and he cannot tolerate such disrespect. Therefore, he feels compelled to stand up for Romeo and defend his honor by fighting Tybalt himself.

Secondly, Mercutio has a deep loyalty and friendship with Romeo. Throughout the play, Mercutio demonstrates unwavering support and affection for Romeo, often serving as a mediator between Romeo and his internal conflicts. In this particular scene, Romeo's refusal to fight Tybalt is seen as a sign of weakness or cowardice by Mercutio. Mercutio believes that Romeo should defend his honor and that of their families by fighting Tybalt, even if it means risking their lives. Unable to bear the thought of Romeo being dishonored or viewed as weak, Mercutio steps in to protect his friend's reputation.

Lastly, Mercutio's actions can be interpreted as driven by his own stubbornness and pride. In the heated exchange between Tybalt and Mercutio, their words escalate, and Tybalt insults Mercutio by calling him a "ratcatcher." Mercutio, being insulted and challenged, cannot back down without losing face or appearing weak himself. His pride pushes him to draw his sword on Tybalt and engage in a fight to prove his own worth and honor in the eyes of others.

Overall, Mercutio's decision to draw his sword on Tybalt stems from a combination of his hot-headed nature, his loyalty to Romeo, and his personal pride. He sees an opportunity to defend his friend and himself while satisfying his need for honor and validation.