Many see Romeo as very immature and fickle, especially when it comes to love. At the beginning of Romeo and Juliet, Romeo is extremely dejected because Rosaline, the girl he loves, has vowed celibacy and rejected his love. He hasn't eaten in days, locks himself in a dark room, and avoids his friends and relatives. In this case, Romeo sees love as bitter-sweet.
Love is a smoke rais'd with the fume of sighs;(190)
Being purg'd, a fire sparkling in lovers’ eyes;
Being vex'd, a sea nourish'd with lovers’ tears.
What is it else? A madness most discreet,
A choking gall, and a preserving sweet. (1.1)
However, that night, he easily forgets about Rosaline and falls in love with Juliet at first sight ("Did my heart love till now? Forswear it, sight! For I ne'er saw true beauty till this night."). Romeo's love for Juliet is impulsive. He vows his love eternally the very same night they meet before even speaking to her and marries her the following day.
Romeo is in love with love. He lets love preoccupy him and takes it to an excess that both his father and his good friends Benvolio and Mercutio notice. As Romeo pines with unrequited love for Rosaline at the play's start, Benvolio and Mercutio try to cajole him out of it, to persuade him to put love into a more reasonable perspective. Benvolio urges Romeo to attend the masquerade, pointing out that there are many beautiful young women in Verona other than Rosaline to choose from. Both he and Mercutio would like to see Romeo enjoying love rather than moping around in passionate dejection. Alas, if the friends meant Romeo to discover love's light-hearted side at the masquerade, that plan backfired: Romeo does forget Rosaline but only to fall wholly head-over-heels in love with an even more dangerous object. Juliet, from the rival house of Capulet, is truly a forbidden love. Romeo, still in love with love, has no restraint in his relationship with Juliet: he must marry her immediately, heedless of the consequences.