The Capulets and the Montagues are fighting. The Prince tells them to stop. The Prince threatens to kill them if they don't listen.
Stop fighting in my streets!
I wish Rosaline did not turn me down. I love her so much.
Why are you sad Romeo?
I am sad because Rosaline turned me down.
Forget her and come to the ball with us.
Lord Capulet .... can I marry your daughter?
Juliet has to agree to the wedding.
At the party, Tybalt heard Romeo's voice. Tybalt wanted to kill him. Lord Capulet didn't want Romeo to be killed because "Romeo is a good man." (Also, if Lord Capulet says yes to kill him, he will die from the Prince.)