In Shakespeare's Romeo and Juliet, the use of a five-act play is based upon its dramatic structure. In the late 19th Century, Gustav Freytag, an novelist and playwright, identified the form used in five-act plays. Freytag's work included the diagram originally referred to as the Freytag pyramid, now often called a plot diagram.
Although Freytag's analysis of dramatic structure is based on five-act plays, it can be applied (sometimes in a modified manner) to short stories and novels as well.
In terms of Romeo and Juliet, this form would have been followed (as Shakespeare was a English Renaissance playwright and wrote in the form popular in that day).
Act One of the play introduces our main characters; Juliet's parents think she should marry, and recommend Paris. This is also the scene where Romeo and Juliet meet and fall in love, and while the major conflict has not yet arisen, there is foreshadowing with regard to Tybalt's behavior. This is the introduction.
In Act Two, Juliet and Romeo speak at the famous balcony scene. They agree to marry. Tybalt has called Romeo out to duel. (Here we seeing the rising action, the development of serious conflicts, and the heightening of the dramatic intensity of the story.) Returning to the friar's cell, the young people marry.
In Act Three, tragedy strikes quickly as Romeo kills Tybalt after he kills Mercutio.
In Act Four, Juliet is being forced to marry Paris and takes the friar's potion to make her sleep like dead. Thinking she is dead, the family puts her in the burial vault. Romeo has been banished for killing Tybalt. On the wedding day, the family finds Juliet "dead."
The scenes in each act organize the play's action. The first four acts contain five scenes, each intricately placed to build the dramatic action and enable the audience to care about the young lovers. The five scenes in each act allow Shakespeare to "spin his yarn." The audience becomes engaged, and now agitated waiting to see how the conflicts introduced will be resolved.
Act Five moves swiftly: we have the facts, and now the resolutions comes about to answer questions and alleviate the audience's "fears." Mistakes lead to tragedy:
How now, Balthasar?
Dost thou not bring me letters from the friar?
How doth my lady? Is my father well?
How fares my Juliet? That I ask again,
For nothing can be ill if she be well. (12-16)
Balthazar erroneously reports that Juliet is dead. Romeo visits a druggist to purchase poison, for he will join her that night. Then in scene two, Friar Lawrence realizes that Romeo never got his message about Juliet's "fake" death, spelling potential disaster. In the third scene, Romeo arrives, he and Paris fight and Paris dies. Romeo thinks his wife is dead and kills himself. Friar Lawrence arrives late. Juliet wakes and refuses to leave Romeo, and kills herself.
Yea, noise? Then I'll be brief. O happy dagger!
Snatches Romeo's dagger.
This is thy sheath; there rust, and let me die.(174-175)
She stabs herself and falls on Romeo's body.
All that is left is that their story be told. This act must be swift to give the sense of the brevity of love and life. Had it been any longer, the audience would have lost interest (as plays were already very long at the time) and the swiftness of loss for the Montagues and Capulets reinforces the idea that life is short and fighting over foolish things brings us too soon to death with too many regrets.