There are two examples of unrequited love in Romeo and Juliet: Juliet does not reciprocate the love Paris feels for her, and Rosaline does not love Romeo.
Firstly, Juliet refuses to marry Paris even though her parents insist on the marriage between the two young characters. In Act III, Scene 5, Juliet tells Lady Capulet that she will not marry Paris.
Now by Saint Peter's Church, and Peter too,
Rather than Paris. These are news indeed!
He shall not make me there a joyful bride!
I wonder at this haste, that I must wed
Ere he that should be husband comes to woo.
I pray you tell my lord and father, madam,
I will not marry yet; and when I do, I swear
It shall be Romeo, whom you know I hate,
Juliet's courage in acting against her parents wishes is significant because it reflects the maturity of her character. Her love for Romeo is also exhibited in this quote. Although Paris experiences rejection from Juliet, he is not alone.
Romeo's love for Rosaline is not reciprocated. Rosaline is uninterested in Romeo and has decided to remain chaste. After Romeo meets Juliet, he visits Friar Laurence to ask him if he can marry the two young lovers. However, Friar Laurence is surprised to hear the news of Romeo's sudden love for Juliet. Friar Laurence believes Romeo is acting in haste. Romeo defends his love for Juliet by comparing it to his love for Rosaline.
I pray thee chide not. Her I love now
Doth grace for grace and love for love allow.
The other did not so.
Romeo's comparison of his experience with Juliet as opposed to his experience with Rosaline is significant because it reveals Romeo's impulsive character. As well, the two different forms of love, unrequited and romantic, foil each other. Romeo and Juliet's love appears stronger partly because it contrasts with the unrequited love in the play.