What are the reasons why Tybalt hates Romeo in Shakespeare's Romeo and Juliet?

Tybalt is a Capulet and the cousin of Juliet in Shakespeare's play Romeo and Juliet. Romeo is a Montague and, for a reason never given, the two families are embroiled in a bitter feud which often spills over into the streets of Verona. In Act I, Scene 1, Tybalt expresses his hatred for the Montagues when he confronts Romeo's cousin Benvolio after a disagreement among Montague and Capulet servants. Tybalt says,

What, drawn and...

Tybalt is a Capulet and the cousin of Juliet in Shakespeare's play Romeo and Juliet. Romeo is a Montague and, for a reason never given, the two families are embroiled in a bitter feud which often spills over into the streets of Verona. In Act I, Scene 1, Tybalt expresses his hatred for the Montagues when he confronts Romeo's cousin Benvolio after a disagreement among Montague and Capulet servants. Tybalt says,

What, drawn and talk of peace? I hate the word
As I hate hell, all Montagues, and thee.
Have at thee, coward!

At Capulet's party later in Act I, Tybalt overhears Romeo speaking. Romeo, Benvolio and Mercutio have crashed the party and are wearing masks to hide their identities. Tybalt is enraged that a Montague would have the temerity to intrude on a gathering held by their enemy. Tybalt intends to confront Romeo when Capulet, who doesn't want his party spoiled, stops him. Tybalt is not to be put off and vows to get revenge. He says in Act I, Scene 5,

Patience perforce with willful choler meeting
Makes my flesh tremble in their different greeting.
I will withdraw, but this intrusion shall,
Now seeming sweet, convert to bitt’rest gall.

Tybalt sends a letter to Romeo challenging him. When they meet, in Act III, Scene 1, Tybalt is ready to fight. He says,

Romeo, the love I bear thee can afford
No better term than this: thou art a villain.

In a case of dramatic irony, Tybalt doesn't know that Romeo has, a few hours earlier, married Juliet and is know his in-law. Romeo tries to avoid the fight by telling Tybalt he actually loves him. Romeo says,

I do protest I never injured thee
But love thee better than thou canst devise
Till thou shalt know the reason of my love.
And so, good Capulet, which name I tender
As dearly as mine own, be satisfied.

Mercutio, who also doesn't know Romeo's motives, hates that his friend has backed down and he fights Tybalt in his place. When Romeo tries to break up the fight, Mercutio is stabbed and dies. Romeo then becomes "fortune's fool" and seeks revenge. He kills Tybalt, and the remainder of the play plunges Romeo and Juliet toward their final tragedy.