A prevalent device in Shakespeare's Romeo and Juliet is the oxymoron. Remember, an oxymoron is two words with contradictory meanings. For example: Jumbo Shrimp or bitter sweet. Authors often use oxymoron to show contradiction in thoughts or ideas. Shakespeare uses them for similar reasons in his tragedy.
In Act I scene i Romeo says:
Why then, O brawling love! O loving hate!
O any thing, of nothing first create!(175)
O heavy lightness! serious vanity!
Misshapen chaos of well-seeming forms!
Feather of lead, bright smoke, cold fire, sick health!
This represents several uses of oxymoron at play. "Loving hate" seems to contradict itself. If you love someone do you also hate them? "Heavy lightness" suggests that something is both heavy and light. "Feather of lead" brings the contradiction between how much a feather weighs and how much lead weighs. Fire is hot, but the use of the oxymoron "cold fire" just like "sick health" brings in these two opposing ideas. All of these show the contradictions occurring in Romeo's mind as he tells Benvolio of his unrequited love of Rosaline.
In Act II scene ii Juliet speaks perhaps the most famous oxymoron of the play:
Good night, good night! Parting is such sweet sorrow
The oxymoron "sweet sorrow" brings together two differing ideas of sweetness and sorrow. How can leaving her beloved be both sweet and sorrowful? It is so sweet that she will continue saying it to him over and over "until it be morrow".