Shakespeare grabs the attention of his audience in much the same way as modern screenwriters and movie directors, by including gratutious sex and violence. Remember that The Tragedy of Romeo and Juliet is a drama, a play; it was and is intended to be acted out and viewed by an audience.
The opening scene immediately opens up with characters (Capulet servants Sampson and Gregory) poking fun at each other and making very ribald, baudy, and raunch jokes of a sexual nature.
Sampson: Ay, the heads of the maids, or their maidenheads; take it in what sense thou wilt.
Gregory: They must take it in sense that feel it.
Sampson: Me they shall feel while I am able to stand, and 'tis known I am a pretty piece of flesh.
Gregory: 'Tis well thou art not fish; if thou hadst been Poor John.
In the above exchange, Sampson is bragging about his own "manly prowess" and "manhood", to which Gregory is very quick to put him down. The first 50 lines of the scene contain one put down or insult after another, one sex joke after another. This is, of course, immediately followed by a street fight, complete with swords and violence, and even more derogatory remarks and insults.
The action and suspense hit the audience from the very beginning of the play, and is continually added to in the opening scenes to keep the audience focused and on the edge of their seats. The proclamation by Prince Escalus in lines 82-89 of death to anyone who interrupts the peace of his town, Paris' plea for Juliet's hand in marriage and her father's subsequent refusal in Scene 2, Benvolio's plot to take Romeo to crash the Capulet party see he can compare his current love and lustful feelings for Rosaline to other beauties, and Lady Capulet's plight to convince Juliet to consider Paris as a suitor in Scene 3 all pull the audience into the plot and action of the play, and leave them stuck there, eagerly anticipating the next event.
When Paris asks to marry Juliet, her father replies that
"She hath not seen the change of fourteen years./Let two more summers wither in their pride,/Ere we think her ripe enough to be a bride."
He is telling Paris to be patient and wait until she is older. However, he turns around 10 lines later and invites Paris to the party he is throwing to introduce Juliet to eligible bachelors. And, later in the same scene, when Romeo and Benvolio make the decision to put in an appearance at the party, the audience has no choice but to accept and believe that nothing but trouble and disaster will come of the evening.
Even though the audience already knew the outcome of the play, as they are told about the lovers' death in the prologue to Act 1, they are drawn to the events that lead up to it. It is similar to watching modern-day reality television shows. You know it is going to end badly, but the true entertainment comes in the HOW and the WHY of the events, not the WHAT.