why does romeo refuse to fight tybalt

Romeo refuses to fight Tybalt because he has just secretly married Juliet, who is Tybalt's cousin. This marriage makes Tybalt his family, and Romeo does not want to harm his new relative or escalate the feud between their families. His love for Juliet and this new familial bond lead him to seek peace rather than conflict with Tybalt, despite the long-standing feud between the Montagues and Capulets. Romeo even asks Tybalt to hold off fighting until Tybalt understands why Romeo feels love for him.