why do you think Shakespeare begins Romeo and Juilet with a violent street fight?

It certainly hooks the audience in. People love a good violent scene, even in Elizabethan times!

The moments just prior to and into the street fight at the beginning of the play set the stage for the rest of the action. The beginning of this scene provides important background information about the families we'll meet, and it introduces us to each and every component of Verona's society; it also shows us how occassionally the disobedience of servants can marr the honor of their masters....... and Romeo and Juliet abounds with the theme of honor.