At the beginning of the play, Romeo is in love with Rosaline. We do not learn much about this mysterious girl, but Romeo is obviously smitten with her. Unfortunately for him, Rosaline has sworn not only not to fall in love, but to remain chaste:
She'll not be hit
With Cupid's arrow. She hath Dian’s wit,
And, in strong proof of chastity well arm'd,
From Love's weak childish bow she lives unharm'd.
It is not clear that Rosaline is committed to her chastity. Perhaps she is just not interested in Romeo. Friar Lawrence suggests that perhaps she (correctly) did not view Romeo's professions of love as completely sincere:
O, she knew well
Thy love did read by rote, and could not spell.
In any case, Rosaline's spurning of his love leaves Romeo devastated, perhaps near suicidal, at the beginning of the play, and Benvolio and Mercutio are both determined to bring him out of his malaise, which is apparently one of the reasons they attend the Capulets' masque. Still, the Rosaline affair raises questions in the minds of the audience about the sincerity of his love for Juliet. These reservations are quickly resolved, but it is clear that Shakespeare intends to suggest that young love is often impulsive and ephemeral.