Final answer:
Romeo tells Tybalt he must love him due to his secret marriage to Juliet, which has caused his love for her to also include her cousin Tybalt, thereby preventing him from fighting.
Step-by-step explanation:
Romeo says to Tybalt, "The reason that I have to love thee Doth much excuse the appertaining rage To such a greeting. Villain am I none. Therefore farewell. I see thou knowest me not." because at this point in Shakespeare's play Romeo and Juliet, Romeo has secretly married Juliet, who is Tybalt's cousin. Romeo's love for Juliet has extended to her family, including Tybalt, even though Tybalt is not aware of the marriage and remains hostile due to the feud between their families, the Montagues and the Capulets. Consequently, Romeo refuses to fight Tybalt and expresses this sentiment despite Tybalt's insults, attempting to avoid further conflict.