The reason for Tybalt's hatred of Romeo is actually quite elementary. William Shakespeare's The Tragedy of Romeo and Juliet is about two young lovers desperate to be together but ultimately undone by the bitter, ancient feud that divides their families. Romeo is a Montague; Juliet is a Capulet. These two prominent families have maintained this violent feud for so long that the contemporary principals of each family no longer really know why the feud exists. Indeed, Lords Montague and Capulet seem to have mellowed somewhat by the time the story takes place. With respect to Tybalt, however, all that matters is that the Montagues and Capulets despise each other, and Tybalt is a Capulet. Note in the following passage from Act I, Scene I of Shakespeare's play Tybalt's ingrained hostility towards anything or anyone associated with the Montague clan:
TYBALT What, art thou drawn among these heartless hinds?
Turn thee, Benvolio, look upon thy death.
BENVOLIO I do but keep the peace: put up thy sword, Or manage it to part these men with me.
TYBALT What, drawn, and talk of peace! I hate the word,
As I hate hell, all Montagues, and thee:
The reason for Tybalt's hatred of Romeo runs no deeper than the fact of the long-running feud. Tybalt's antipathy towards Romeo, as well as towards all Montagues, is pervasive throughout the play. Again, in Act I, Scene V, Romeo, having sneaked into the Capulet's ball, is unmasked and confronted by Tybalt. Responding to Lord Capulet's suggestion that the interloper be left alone, Capulet having spoken highly of Romeo ("Verona brags of him; To be a virtuous and well-govern’d youth"), the patriarch's brash and violent nephew states instead, "Uncle, this is a Montague, our foe, A villain that is hither come in spite,To scorn at our solemnity this night."