Act I, Scene 5 is where Romeo and Juliet meet and instantly fall in love. They also discover through the Nurse that each of them has fallen in love with their enemy. The two lovers go ahead and plan a life together in spite of this, showing how intense their love for each other is. This event sets the events of the play into motion.
It is also in this scene of Act I where Tybalt recognizes Romeo's voice at the Capulet party. Displaying his hot-headed temper, Tybalt wants to immediately get his sword and fight Romeo for daring to crash the party. Lord Capulet doesn't allow Tybalt to confront Romeo, so Tybalt swears vengeance on Romeo. This oath of revenge foreshadows Mercutio's death and Romeo's banishment. The fight between Mercutio and Tybalt leads to the fight between Romeo and Tybalt. As a result, Romeo is banished, and the two young people die tragically.