Because it's a pair of teenagers, for all intents and purposes. Neither are emotionally equipped to deal responsibly with those sorts of emotions and so in the course of days, they are 'irrevocably in love'.
Friar Laurence: (
to Romeo, who has just explained his love for Juliet whom he met the previous night and now plans to marry)
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.
–
Romeo & Juliet, Act II, scene iii.
Teenagers falling in love at the drop of a hat is kind of a staple of fiction. Of course, Shakespeare had enough sense to know that the old trope would probably end up pretty damn badly, which is one of the reasons the full title of his most famous example of the trope is
The Most Excellent and Lamentable Tragedy of Romeo and Juliet.
But people aren’t buying tickets to
Battle Angel for either a realistic romance or a unrealistic-but-fun romantic film style romance. We want to watch cyborgs beat the chrome out of each other.
