The play does not say what caused the feud between the Montagues and the Capulets. The Prologue speaks only of an old enmity between two families of equal nobility, and what the text shows are its effects.
In Act I, scene 1, the fight begins between servants of the two houses, who provoke each other in a square before their masters appear. Benvolio tries to part them; Tybalt challenges him, saying he hates the word peace as he hates hell; and Capulet and Montague come to blows. Prince Escalus says that three fights have been born of a careless word, and threatens death to anyone who again disturbs the city's peace.
The feud reaches the party: at the ball (Act I, scene 5), Tybalt recognizes a Montague and wants to kill him, and Capulet restrains him, saying Verona is proud of Romeo and that he does not want trouble in his house. Paris laments that the two old men have lived so long in discord (Act I, scene 2). For the lovers, the feud becomes an obstacle: Romeo's name is what separates them (Act II, scene 2).
In the end, the feud costs five deaths on stage, Mercutio, Tybalt, Paris, Romeo, and Juliet, and Montague adds the death of his wife, killed by grief over their son's exile. The Prince says that he too was punished for having closed his eyes to the brawls, since he lost two kinsmen, and Capulet holds out his hand to Montague before the tomb, with the promise of a golden statue of Juliet and another, equally rich, of Romeo (Act V, scene 3).