What is the purpose of Act I in the play, Romeo and Juliet?

As in all dramas, the characters and the
conflicts/problems are introduced to the audience in the first act, which contains the
exposition and the initial action, and Act I of Shakespeare's Romeo and Juliet
is no exception.  Of course, the Prologue provides the Elizabethan audience a
summary of what it to come in the play, despite the fact that audiences were probably
familiar with the story of Romeo and Juliet since it was already known and very popular
when Shakespeare came to write the play around 1594.

In Act
I the antipathy of the two households, Capulet and Montague, is introduced by the
exchange of insults among servants from both families.  To underscore the acrimony of
these families, the old Lords Capulet and Montague come out to the street and insult
each other as well. As a result of the renewing of the feud, the Prince of Verona issues
an ultimatum to anyone who breaks the peace.  Then, Romeo is presented as the
melancholic victim of unrequited love. For, Rosalind has rejected him and gone to the
convent, leaving Romeo to wallow in self-pity.  His good friend Benvolio suggests
that

One pain
is lessened by another's anguish...

One deperate grief
cures with another's languish.
(1.2.)

Thus, he encourages
Romeo to mask himself and join him and Mercutio as they sneak into the Capulet's party
for their daughter Juliet. This fateful action of going is the catalyst for Romeo's
being starstruck by the beautiful Juliet and is, thus, pivotal to all the following acts
of the play