In act 1, scene 2 of William Shakespeare's Julius Caesar, Cassius is talking with Brutus on a street in Rome, trying to discover what's troubling Brutus, when a crowd of Roman citizens in another street raises a shout in praise of Caesar. Brutus remarks,
BRUTUS. What means this shouting? I do fear the people
Choose Caesar for their king (1.2.84–85).
This gives Cassius an opportunity to confide to Brutus that he, too, shares Brutus's concerns about Caesars ambitions. It also gives him the opportunity to remind Brutus that one of his ancestors, Lucius Julius Brutus, was instrumental in expelling the last king of Rome and became a founder of the Roman Republic almost 450 years ago. Since that time, no king has ruled over Rome, but Caesar is now giving every indication that he wants to be made king of Rome.
In February of 44 BCE, a month before the first scenes of Julius Caesar occur, Caesar declares himself "Dictator perpetuo," meaning "Dictator for Life." Brutus and Cassio learn from Casca that Caesar has just performed a deft bit of political theatre in front of a large crowd of Roman citizens by seeming to reject a crown offered to him by his friend Marc Antony, which only incites the crowd to urge Caesar to accept it.
Brutus invites Cassius to dinner the next evening so that they can discuss the matter further. Early the next morning, however, Cassius appears at Brutus's home with five other men, whose intent is to convince Brutus to join them in assassinating Caesar.
Brutus doesn't need much convincing. He's already wrestled with his own personal feelings about Caesar—in act 1, scene 2, Brutus tells Cassius that he's been "with himself at war" (1.2.51) about it— and he's thought about the issues of Caesar's unrestrained ambition and his increasing danger to the Roman Republic.
Brutus decides to join the conspirators and assumes the responsibility of organizing them for the assassination.
The reason why Brutus joins with the other conspirators is that Caesar's ambitions, his rise to nearly absolute power, his popularity with the people of Rome, his rejection of Roman institutions—such as the Senate, which he treats with contempt—and his desire to be king, all pose a serious and immediate threat to the Roman Republic. Brutus explains this succinctly in his oration to the people of Rome after Caesar's assassination.
BRUTUS. If there be any in this
assembly, any dear friend of Caesar's, to him I say that
Brutus' love to Caesar was no less than his. If then that
friend demand why Brutus rose against Caesar, this is my
answer: Not that I loved Caesar less, but that I loved Rome