Originally, Romeo's and Juliet's relationship was one of enmity as the house of Montague was in a feud with the house of Capulet, though the Prince had forbidden feuding. At the Capulet's ball, at which Romeo and his friends made themselves guests, Romeo and Juliet developed a relationship of flirtation that became a relationship of young impassioned love after their rendezvous at the balcony, a relationship that is later confirmed when Nurse seeks a statement of intent from Romeo.
Because of the feud, Romeo and Juliet had not even met prior to the ball at the Capulets; Romeo wouldn't even have been there to meet Juliet except for a passing servant and a handbill of invitation that chanced Romeo's way when the servant asked if he could read.
ROMEO: A fair
assembly; whither should they come?
SERVANT: Now I'll tell you without asking: my master is the
great rich Capulet; and if you be not of the house
of Montagues, I pray, come and crush a cup of wine.
Rest you merry!
When Romeo does espy Juliet from across the room, his heart is moved as though he had never loved before and poor fair Rosaline is cast off before Juliet's charm:
ROMEO: O, she doth teach the torches to burn bright!
It seems she hangs upon the cheek of night
Like a rich jewel in an Ethiope's ear;
Beauty too rich for use, for earth too dear!
Did my heart love till now? forswear it, sight!
For I ne'er saw true beauty till this night.
Their relationship changes from one of momentary flirtation when Romeo defies the dangers of the feud and breaches the Capulet's orchard wall to stand in her moonlit radiant light.
ROMEO: Can I go forward when my heart is here?
Turn back, dull earth, and find thy centre out.
He climbs the wall, and leaps down within it
Their early relationship takes one more turn the next day when Juliet sends Nurse to ascertain Romeo's true intent and, as his intent is marriage, next follows their conspiring with Friar Laurence to marry them. First, Juliet forewarns Romeo that she shall send for his answer on the morrow:
JULIET: If that thy bent of love be honourable,
Thy purpose marriage, send me word to-morrow,
By one that I'll procure to come to thee,