The Petrarchan conceit, an exaggerated description of love in terms of its pleasurable pains reached its height during the Renaissance. Thus, when Romeo speaks in contradictory terms of his unrequited love for Rosalind in Act I, the Elizabethan audiences would quickly recognize this manner of speech. With this expression in the terms of courtly love in which the lover addresses an indifferent noblewoman, Romeo introduces also the theme of the interrelationship of opposites that is prevalent throughout the play.
This "much to do with hate, but more with love" also introduces the ensuing action of the first act in which Cupid, who "muffle[s]" the view of love, as Romeo declares in his Petrarchan conceit, causes Romeo to fall blindly in love with Juliet, whom in contrast to Rosaline as a moon goddess, he perceives as the sun. In Scene 5, he begins to speak in extremes again:
Did my heart love till now? forswear it, sight!
For I ne'er saw true beauty till this night. (1.5.54-55)
Romeo's falling in love with Juliet, then, is part of the theme of contrasts as his melancholy of Scene 1 in his Petrarchan conceit is quickly dispelled. Clearly, Romeo is the embodiment of the Renaissance ideal of courtly love as he speaks both of Rosaline and of Juliet.
An Oxymoron is any phrase with contradictory parts. A popular joke is describing "Government Intelligence" as an oxymoron; the idea is that the two terms are mutually exclusive and so cannot be true.
In Romeo and Juliet, there are many oxymorons throughout the text. In his first appearance, Romeo uses a number in speaking with Benvolio:
Romeo: Why, then, O brawling love! O loving hate!
O anything of nothing first create!
O heavy lightness! serious vanity!
Mis-shapen disorder of perfectly pleasing forms!
Feather of lead, bright smoke, cold fire, sick health!
Still waking sleep! That is not what love is!
(eNotes eText, Act 1 Scene 1, Modern)
The italicised phrases are all oxymoronic in nature, each being a description of a thing in opposing terms. For example, "Loving Hate" is contradictory in nature; surely the two feelings cannot be felt together? "Sick Health" and "Waking Sleep" are also contradictory, as the presence of one would naturally preclude the other.
Romeo, then, is using the confusing contradictions to attempt a verbal reconciliation of his love for Juliet, whose family is in bitter feud with his own. She is forbidden to him, and yet he loves her; love has no rational explanation, and neither do these oxymorons. Essentially, his use of the oxymoronic is metaphorical for his own emotional state: he knows that he must hate Juliet for her heritage, but he also knows that he loves her for herself; "Brawling Love" is his first oxymoron and perfectly describes his own emotional and cultural quandary.
at that point in the play romeo had not met juliet