Why does Tybalt hate the Montagues so much in act 1 of "Romeo and Juliet"?
The short answer is that we are never told. The Prologue to the play that the "two households" of Capulet and Montague have an "ancient grudge" but Shakespeare never explains what that grudge is or why it started. It may be that Tybalt as a Capulet is simply continuing the grudge for its own sake with no other motive. Certainly in the Masque scene, he accuses Romeo of have attended to "fleer and scorn" at his family and vows to kill him "by the stock and honour of my kin". This would imply that Tybalt's anger is directed at some perceived slight to his family honour.
In Act 3 scene 1 Tybalt tells Romeo that he cannot excuse the "injuries" done to him, which may be a reference back to the masque or some other undisclosed incident.
Another answer is more psychological and character-driven. In Act 1 Tybalt tells Benvolio that he hates the word peace "As I hate hell, all Montagues and thee" and certain productions have presented Tybalt as an almost psychotic character who enjoys and revels in the violence and aggression of the grudge. Baz Luhrman suggests this in his film version.