In Romeo and Juliet, Act 3, Scene 1, many important events take place that drive the rest of the events in the play. In Act 3, Scene 1, Mercutio and Benvolio are out in the streets of Verona, when the angry Tybalt and his comrades confront Mercutio. They insist that Mercutio and Benvolio are friends with Romeo, and demand to know where they can find him. Tybalt is in a rage over finding Romeo at the Capulet party. Benvolio even warns the men to take the argument elsewhere, because they are in the wide open streets where anyone can view the discord.
It is then that Romeo walks over, and places himself into the conversation between Mercutio and Tybalt. Romeo actually tries to extricate himself from the argument with the enraged Tybalt, but to no avail. Tybalt is ripe for a fight over Romeo's attendance at the masquerade ball, calling him a villain.
Mercutio, being one to engage in fights, challenges Tybalt to a sword fight in defense of Romeo. Mercutio is killed by Tybalt's sword. Romeo then avenges Mercutio by slaying Tybalt. Romeo is then banished from Verona as punishment.
In short form, the three reasons that Act 3, Scene 1, are the most pivotal in the play are 1.) Tybalt kills Mercutio, leading Mercutio to pledge a curse upon both their houses, foreshadowing the tragic ending, 2.) Romeo kills Tybalt, cousin to Juliet, thereby sealing his fate by being banished, making it almost impossible for the lovers to successfully be together, and 3.) Lady Capulet demands that the Prince punish Romeo for his crime. Lady Capulet is instrumental in the Prince's decision to banish Romeo.
These three actions propel the rest of the play, as all actions that the characters take lead to tragedy. If the events in Act 3, Scene 1, would have played out peacefully, then this would have altered the tragic tone of the play, and would have changed the entire story. This scene shows that fate or free will deals an unfair hand to Romeo and Juliet. Then it becomes the responsibility of the reader to interpret the theme of fate versus free will, and how this theme pervades the play.