Are there any quotes in Romeo and Juliet that show passion is unpredictable?

The unpredictability of love and passion is a recurring theme in this play. When we first meet Romeo, he is pining for Rosaline. Yet when he first sees Juliet, he forgets all about Rosaline:

Did my heart love till now? forswear it, sight!

Later in that same scene (Act I, Scene 5), Juliet is informed of Romeo's family affiliation, and she laments the ironic coincidence that she should have fallen in love with someone who...

The unpredictability of love and passion is a recurring theme in this play. When we first meet Romeo, he is pining for Rosaline. Yet when he first sees Juliet, he forgets all about Rosaline:

Did my heart love till now? forswear it, sight!

Later in that same scene (Act I, Scene 5), Juliet is informed of Romeo's family affiliation, and she laments the ironic coincidence that she should have fallen in love with someone who is supposed to be her enemy.

My only love sprung from my only hate!
Too early seen unknown, and known too late!
Prodigious birth of love it is to me,
That I must love a loathed enemy.

In Act II, Scene 1, Mercutio makes reference to the unpredictability of love when speaking of the passions of Romeo. He mentions the legend of the King and the Beggar-Maid, in which a king falls in love with a woman he sees begging on the streets:

Young Adam Cupid, he that shot so trim,
When King Cophetua loved the beggar-maid!

Cupid's arrow shot the king, and produced this unlikely, unpredictable passion.

Next, during the balcony scene (Act II, Scene 2), Juliet expresses her concern to Romeo that their passion might have erupted too suddenly. It's like the lightning, and can't be entirely trusted. It might suddenly "cease to be":

I have no joy of this contract to-night:
It is too rash, too unadvised, too sudden;
Too like the lightning, which doth cease to be
Ere one can say 'It lightens.'

When Friar Laurence learns of Romeo's new love interest, he comments on the fickleness or unpredictability of young men's passions (Act II, Scene 3):

Holy Saint Francis, what a change is here!
Is Rosaline, whom thou didst love so dear,
So soon forsaken? young men's love then lies
Not truly in their hearts, but in their eyes.

And a bit later, in Act II, Scene 6, Friar Laurence warns Romeo that his intense passion is liable to die out suddenly and violently:

These violent delights have violent ends
And in their triumph die, like fire and powder,
Which as they kiss consume…