The Chorus states that since Romeo and Juliet's families are involved in a feud, they will never be able to marry. Analyzing this specific portion of the text shows Romeo and Juliet trying to get revenge on their parents for this feud. By secretly loving one another, they're attempting to get revenge on their parents and make them realize that they can bring together both sides.
Being held a foe, he may not have access To breathe such vows as lovers use to swear.
And what love can do, that dares love attempt. Therefore thy kinsmen are no stop to me.
If they do see thee, they will murder thee.
The Capulets [Juliet's family] have a feud with the Montagues [Romeo's family]. Due to this, if Juliet's family discovers that Romeo visits Juliet, they will take revenge on the Montagues by killing him.
Peter, the Nurse's husband claims that if any man dared to insult his wife, he would take revenge on them and his sword would have been out immediately. This shows a motif of revenge as Peter would take revenge on anyone who dared say something bad about the Nurse. He would get them back to make them pay.
I saw no man use you at his pleasure. If I had, my weapon should quickly have been out.