One of the primary themes of Romeo and Juliet is the strife between the houses of Capulet and Montague, which would not allow the two young lovers to ever have a happy ending together. The Chorus' prologue at the very beginning tells us that they will end their lives because of the hatred between their two families:
Two households, both alike in dignity,
In fair Verona, where we lay our scene,
From ancient grudge break to new mutiny,
Where civil blood makes civil hands unclean.
From forth the fatal loins of these two foes(5)
A pair of star-cross'd lovers take their life;
Whose misadventur'd piteous overthrows
Doth, with their death, bury their parents’ strife.
The fearful passage of their death-mark'd love,
And the continuance of their parents’ rage,(10)
Which, but their children's end, naught could remove,
Is now the two hours’ traffic of our stage;
The which if you with patient ears attend,
What here shall miss, our toil shall strive to mend.
Because of this strife, it makes sense that Shakespeare would have started the play (immediately following the Chorus) with a fight between servants of the houses of Capulet and Montague. He is demonstrating how all-pervasive this feud is, not only for the members of the family, but also for their serving staffs. This also gives the audience a chance to hear the Prince get involved and tell these warring sides to stop, or else!
Why not? Isn't it such an exciting and dramatic start to the play.
Remember, Elizabethans were going to the theatre to be entertained, much in the way that you might go to the cinema or to a musical today.
If a play had no action some people would get bored and feel that the story lacked a sense of real life.
English society was much closer to violence back then. There was no police force and arguments might often end up in violence. Famously, one of Shakespeare's play writing contemporaries, a man called Marlowe, was stabbed and killed in a fight in a pub, just about the time that Shakepeare was writing Romeo and Juliet.
This is also an exciting opening to the play because there are so many people on stage and there is so much to see.
From a plot point of view the fight scene also makes us very aware of the rivalry which exists between two Veronese families at the very start of the play. Remember it is this rivalry which will make it impossible for Romeo and Juliet to just tell their parents that they love each other and want to marry. It is this rivalry that leads directly to tragic consequences at the end of the play - to the death of the lovers who are driven to extreme remedies to try and find a future together.
The reason for beginning the play with brawl between the Montagues and Capulets is that Shakesphere wants to explain the enemity betweenthe two families.
It is fitting that Shakespeare starts the play Romeo and Juliet with a brawl to set the stage got a tragedy. Right after the brawl, the tragedy escalates and climaxes with the double suicide of both Romeo and Juliet
In the Prologue, Romeo and Juliet are referred to as, "A pair of star-crossed lovers" which is the essence of the tragedy. Without he brawl and the subsequent murders, they would not have been "star-crossed". There would be no tragedy and essentially no play.
The brawl also provides with a lively opening to capture the interest of the audience since brawls were common during Shakespearean England.
By starting the play with a brawl, Shakespeare not only establishes the feud between the houses of Montague and Capulet, but sets the stage for the entire restof the play, as far as fate goes.
Remember that fate is the central idea of Romeo and Juliet. The Prince states:
"If you ever disturb our streets again,
Your lives shallpay the forfeit of the peace."
Later on in the play, Mercutio and Tybalt get into a brawl which results in Tyblt killing Mercutio, one of Romeo's best friends. Romeo avenges his friend's life by killing Tybalt. Technically Tybalt should have been killed for his crime, as the Prince had declared, but Romeo took care of that. Still, murder is murder, and Romeo must be punished. Therefore he is banished which, if you recall correctly, ultimately leads to his and Juliet's untimely demise when Friar John is unableto deliver the message about Juliet's taking the sleeping potion to Romeo. Romeo therefore believes Juliet to be dead and kills himself, and upon waking up Juliet sees her lover's deadbody and stabs herself in the chest.
In short, if the brawl between Samson, Gregory and the Montague's servants, Tybalt would not have had to be killed, Romeo would not have been banishedand he and Juliet would not have taken their own lives. Remember that Shakespeare describes them as 'a pair of star-cross'd lovers.'
Fate is the biggest theme of Romeo and Juliet, and the openeing brawl defined the lovers' fates more than perhaps any other act in the play.