In Act 1, Scene 5 of Romeo and Juliet, what language technique is used in Juliet's phrase, "My only love sprung from my only hate!"?

Antithesis puts two opposite ideas together in a grammatically balanced way to highlight a contrast or paradox. The quote "My only love spring from my only hate!" is therefore an example of antithesis. The two sides of the statement are in balance, with the only difference being the substitution of the word "hate" for "love." Juliet expresses through antithesis the seeming paradox that her love and her hate spring from the same source.
Juliet makes this exclamation in Act I, scene v, at the masked ball. She has already fallen in love with Romeo, but she has no idea who he is. She asks her nurse to find out. When the nurse comes back with the information that Romeo is a Montague, Juliet, surprised and dismayed, makes her statement. In it, she encapsulates the idea that her family hatred of all Montagues, the only people she hates, has collided with her love of a Montague, Romeo. In a pithy way, she has summed her dilemma and the conflict that will face both her and Romeo throughout the play, as family loyalty is pitted against true love.

When Juliet utters this line, she is referring to the fact that she feels herself to be in love with this handsome stranger she kissed at her father's party, but she has just learned that he is Romeo Montague, the son of her father's great enemy.  It is a paradox.  A paradox is a statement or situation that seems to be contradictory and therefore appears to be an impossibility; however, there is always some crucial piece of information needed to resolve and understand the paradox.  How can one's only love possibly be embodied by the same person who represents one's only hate?  It seems like an impossibility because a person cannot both love and hate someone else at the same time.  However, the feud between the Montagues and Capulets helps us to understand.  Juliet doesn't really hate Romeo, but she knows that she's expected to hate him because he's a Montague.  She loves him, but she knows that this love is going to be problematic because she is supposed to hate him.  It's a pretty sad irony.