Shakespeare marked the seriousness and intensity of the feelings of people by the formality of the language--the more formal, the more intense. Romeo and his friends are just joking around--their conversation is not particularly serious or intense, so it is in prose.
The nurse
the nurse encourages Juliet to marry Romeo
Blank verse
It's a sonnet.
Generally (but not always!) Shakespeare's characters who spoke in blank verse are the lower-status characters. Think of which characters are not as important, then compare that to some of their speech in Romeo and Juliet.
Romeo doesn't speak in blank verse because he is in an excited and emotional state after meeting Juliet. His dialogue with his friends is informal and reflects his heightened emotions, so he speaks in prose instead of the more structured blank verse.
From the conversation between Benvolio and Romeo's parents, we learn that Romeo is acting melancholic and isolating himself from his family and friends. He is described as spending time alone in his room, refusing to participate in social activities, and exhibiting signs of unrequited love or heartbreak. Overall, Romeo's behavior is characterized by emotional turmoil and withdrawal from his usual social interactions.
The main purpose of Romeo and the Nurse's conversation is for Romeo to find out if Juliet returns his feelings and to arrange for their secret marriage. The Nurse acts as a go-between and provides a way for Romeo and Juliet to communicate.
ABC
Romeo and his friends will escape detection at the party by wearing masks to conceal their identities. This allows them to move freely without being recognized by other guests. They also blend in with the crowd by engaging in typical party activities and conversation.
Mercutio and Benvolio's conversation reveals that Romeo had left the party to wander off by himself, indicating his tendency to isolate himself when troubled. Mercutio also shares his concerns about Romeo's continued melancholy over Rosaline, showing that Romeo's emotional state is a point of worry for his friends.
The nurse
the nurse encourages Juliet to marry Romeo
Mercutio and Benvolio are unaware of the Nurse's conversation with Romeo and engage in banter and teasing with each other. They do not interact with the Nurse during her conversation with Romeo.
Mercutio and Romeo were friends; that's what friends do.
Mercutio and Benvolio are Romeo's friends.
Blank verse