Tybalt sends a letter to challenge Romeo to a duel. Tybalt does this because he seeks revenge. The reason he seeks revenge is because of Romeo being at the Capulet party not too long ago. It shows how he has not forgotten and how his character is focused on revenge.
Tybalt, the kinsman to old Capulet, Hath sent a letter to his father's house.
Romeo will answer it.
A challenge, on my life.
The Nurse gets mistreated by Benvolio and Mercutio when trying to speak to Romeo. They call her a pimp, as to aggravate her. She gets angry and swears that the next time they speak ill of her, she will take them down. She starts to seek revenge on the both thanks to their indecent and rude behaviors.
An he speak anything against me, I'll take him down, an he were lustier than he is, and twenty such jacks.
Romeo and Juliet choose to fall in love despite the consequences that will follow. There is foreshadowing, predicting the negative ending of the story. Romeo and Juliet's rebellious behavior goes against their families and their names.
If they do see thee, they will murder thee.
And what love can do, that dares love attempt. Therefore thy kinsmen are no stop to me.