The juxtaposition of love and hate runs throughout the play, ultimately becoming the source of the conflict that kills Romeo and Juliet.
The prologue establishes the hatred that has existed between the Capulets and the Montagues for many years:
From forth the fatal loins of these two foes
A pair of star-crossed lovers take their life;
Whose misadventured piteous overthrows
Do with their death bury their parents' strife. (Prologue, 5–8)
Here, we learn that two lovers, Romeo and Juliet, will end up committing suicide because of their parents' hatred. Unfortunately, this drastic action is needed to bring and end to their years of "strife."
Before the party where Romeo meets Juliet, he encounters a scene where the Capulet and Montague servants have recently been fighting. Seeing this bloodshed, he laments about the nature of hate:
O me! What fray was here?
Yet tell me not, for I have heard it all.
Here's much to do with hate, but more with love.
Why then, O brawling love, O loving hate... (I.i.178–181)
Romeo uses oxymorons here (such as "loving hate") to demonstrate the powerful forces of both emotions. After all, some people love to hate—they invest just as much mental and emotional energy into nurturing those feelings as others invest into acts of love. Romeo is reflecting on the passions involved in both love and hatred.
In the end, the prince points out to the surviving families that their hatred has brought them unbelievable sorrow:
Where be these enemies?—Capulet, Montague,
See, what a scourge is laid upon your hate,
That heaven finds means to kill your joys with love. (V.iii.301–303)
Because of their ongoing hatred, a "scourge," or punishment, has been inflicted upon both families. "Heaven," or God, has decided to end their ongoing feud by bringing both families to their knees through the death of their children. They now find themselves equal, albeit in the most painful of circumstances. The "joys" of their lives are gone, exposing the truth: they could have avoided such a fate if they had not been consumed by hatred.