In the beginning of the play, we see that Romeo feels he is a slave to love, who bends people "to his will" (Act I, Scene 1). He also feels that love has a contradictory nature; it is composed of complete opposites. We see this in his line "feather of lead, bright smoke, cold fire, sick health, still-waking sleep." We also see in this first scene that Romeo does not think very much of marriage. Romeo is brokenhearted because fair Rosaline has vowed to remain chaste and will not let Romeo come to her bed. Romeo is mistaking feelings of unrequited lust for feelings of love.
However, when he meets Juliet, we see a change in him. In Capulet's orchard he asks Juliet, "O, wilt thou leave me so unsatisfied?," which can be interpreted as a sexual innuendo. However, when Juliet wisely responds, "What satisfaction canst thou have to-night?," meaning to deny him access to her bedroom just like Rosaline, Romeo replies with the sincere line, "The exchange of thy love's faithful vow for mine" (Act II, Scene 2). Suddenly, because Romeo's affection is finally being returned, he is no longer thinking just of lust, but of solid vows.
Hence, at first Romeo mistakes his feelings of lust for true love, but when his love is finally returned, he thinks not just of bedrooms, but of marriage.
Romeo's views on love change with the objects of his love; at first he merely imitates the Petrarchan lover, but after he sees Juliet, Romeo's passionate nature is awakened.
At the start of the play, Romeo is the Petrarchan lover, suffering with great self-awareness, speaking in oxymorons and other exaggerated phrases--"O brawling love! O loving hate!" (Act I, Scene 1, line 149)--attributing the female object of his love with great power over him, a quality unlike the real position of women in fourteenth-century Verona.
Following Romeo's exclamations of love and words of despair and suffering, Benvolio tries to console his friend. After an illiterate servant of the Capulets informs them of the festivities that night, Benvolio encourages Romeo to accompany him to the Capulets' feast, where one more fair than Rosaline will be in attendance. Still suffering the sting of rejection, Romeo agrees to go only so that he might see Rosaline again:
I'll go along, no such sight to be shown,
But to rejoice in splendor of mine own (Act I, Scene 3, lines 90-91).
At the moment that Romeo sees Juliet, his morose ideas end and he abandons his role as the Petrarchan lover. Romeo expresses his appreciation of true beauty as he remarks upon Juliet: "Beauty too rich for use, for earth too dear!" (Act I, Scene 5, line 44).
As he falls in love with Juliet, Romeo quickly desires marriage with her in order to keep her. Romeo's love for Juliet is deeply passionate. When he learns that he is banished because he killed Tybalt, Romeo's depth of feeling causes him to despair. While his love for Juliet is genuine, Romeo's emotionalism drives him to passionate acts.