The purpose of William Shakespeare's inclusion of Rosaline in Romeo and Juliet is open to interpretation. Let us examine some possible explanations for Rosaline's role in the play.
First and foremost, Rosaline, particularly Romeo's infatuation with her, drives the action of the plot. Rosaline is the reason Romeo and Juliet meet. She is the niece of Capulet, which is why she attends the Capulets' ball. Romeo wears a disguise and sneaks into the party, hoping to catch a glimpse of Rosaline, with whom he believes himself to be madly in love. It is at this party that Romeo meets and falls in love with Juliet. The tragic lovers might never have met if Romeo had not first been infatuated with Rosaline.
Romeo's feelings for Rosaline can be interpreted in multiple ways. It can be argued that Romeo's infatuation with Rosaline is intended to be a juxtaposition of his love for Juliet. By including another love interest for Romeo in the play, Shakespeare invites us to examine the similarities and differences between Romeo's relationships with Rosaline and Juliet. Romeo's feelings for Rosaline are immature, unrequited, and based more on physical attraction than anything else. The language he uses when describing his feelings for Rosaline paints a picture of a naïve young man who mistakes lust for love. When he speaks of his love for Juliet, Romeo appears more mature. His feelings for Juliet seem to be deeper than his feelings for Rosaline. Unlike Rosaline, Juliet reciprocates Romeo's affections.
When viewing the play through a less romantic, more practical lens, it can be argued that Romeo's feelings for Rosaline undermine his feelings for Juliet and bring his romantic integrity into question. Shakespeare uses Rosaline to show us how passionate Romeo is. When we first meet him, Romeo is completely consumed by his infatuation with Rosaline, just as he is later completely consumed by his supposed love for Juliet. Romeo believes himself to be enamored of Rosaline. He pines away, longing for her love, yet he is instantly able to forget his feelings for her the moment he lays eyes on Juliet.
This implies that Romeo is fickle and his emotions fleeting. If his feelings for Rosaline are as true as he leads us to believe they are, how is he able to forget about her so quickly? In his relationship with Juliet, Romeo continues to make rash decisions, particularly when he and Juliet rush into marriage. Friar Laurence recognizes Romeo's impulsivity and cautions him not to make hasty, careless choices, such as marrying at such a young age and after knowing Juliet for such a short time, because they can result in undesirable outcomes:
These violent delights have violent ends
And in their triumph die, like fire and powder,
Which as they kiss consume: the sweetest honey
Is loathsome in his own deliciousness
And in the taste confounds the appetite:
Therefore love moderately; long love doth so;
Too swift arrives as tardy as too slow.
Shakespeare's inclusion of Rosaline in the play is an interesting topic for debate, but ultimately, Shakespeare leaves it to the readers to decide for themselves what Rosaline's purpose is and whether or not Romeo's feelings for her undermine his love for Juliet.