During act one, scene one, Sampson and Gregory are on the streets of Verona. As Capulets walk by, they bite their thumbs at them, which was an insult back then. After doing this, the Capulets and Monatagues began to argue, resulting into a huge fight. Although many people including Benvolio tried to break the fight up, they all failed except for the prince. He threatened to kill the next person who started the next fight. If it weren't for him, death may have been brought to someone during that fight, fueling the family fued between the Capulets and Montagues even more.
I will not hesitate to kill any of you!
After a long night of being depressed about Rosaline, Romeo crashes a Capulet party with Benvolio and Mercutio. During this party, Romeo and Juliet spot each other for the first time, falling in love instantly. They ended up dancing for a little, comparing each other to a saint and pilgrim. This set a romantic mood, making them love each other quickly. As they danced, Tybalt recognized Romeo, making him angry that he crashed the party. He was calmed down by the Capulets before he approached Romeo.
Calm down Tybalt!
After the party, Juliet privately thinks about Romeo on her balcony. Even though they had just met, she already seemed to have fell deep in love. She even claims that if he refuses his name as a Montague, she will refuse hers as a Capulet. This shows that the only thing that is keeping Romeo and Juliet from being together, is their names. If it weren't for the family feud they were between, they would be able to love freely.
Oh romeo, please refuse your name, and I shall do the same.
After a lot of planning, Romeo and Juliet finally agree meet up at Friars cell. While Romeo is waiting for Juliet to arrive, the Friar warns Romeo about falling in love too quick. Friar is worried for him because Romeo was just heartbroken over Rosaline's rejection. He says that if someone were to love too quick, it would result in a downfall in the relationship.
Don't fall in love too quick my son!
While the Montagues were walking on the street of Verona, they run into Tybalt. Tybalt is still angry about the Montagues crashing the party which caused him to challenge Romeo to a sword fight. Romeo rejected his this challenge because Tybalt is now Romeo's cousin. Even after this, Mercutio charged at Tybalt, and ended up getting stabbed to death. Romeo's grief was instantly taken over by anger.
Due to Romeo getting payback for Mercutio by killing Tybalt, the prince banished him from Verona. This made him angry because he claims that there is no world outside of Verona. Everything he loves, including Juliet, is in the city that has exiled him. Not only did this make him sad but it also made Juliet worried. Instead of caring about her cousin's death, she only cares about when Romeo will be back.