In act 1 the Capulets and the Montagues got into a huge brawl. The only reason that the fight did not end with someone dead was because of the Prince who broke it up. The Prince said that if the two families were caught fighting again then punishment is death.
Romeo, his cousin Benvolio, and his friend Mercutio had snuck into the Capulet party. The reason for them to sneak into the party. The only reason why he wanted to go there because he saw that Rosaline was on the list. Although when they got there he instantly fell in love with the beautiful Juliet.
At the Balcony scene, Romeo overhears Juliet's private thoughts that were about him. He had jumped over the orchard walls to see her and was spitting off lines of romantic poetry to win her heart.
In Act 3 Romeo was being banished for killing Tybalt. He killed Tybalt out of revenge because he killed Mercutio.