According to Shakespeare's characterization, Brutus is too trusting. His worst mistakes are in trusting Cassius and agreeing to participate in the assassination of Caesar, and later in trusting Marc Antony and agreeing to let him speak at Caesar's funeral. Brutus is portrayed as an introverted, bookish, solitary thinker (similar to Prince Hamlet) who is more interested in philosophy than in practical matters. He simply does not understand people very well. We see this in several things he says. During his big quarrel with Cassius in Act 4, he asks:
Did not great Julius bleed for justice' sake?
What villain touched his body, that did stab,
And not for justice? (4.2)
He not only assumes that Cassius was conspiring against Caesar for the sake of justice, but he actually assumes that all the conspirators were motivated by the same unselfish ideal. Antony, however, has an entirely different read on the conspirators, as he says in the final act while viewing Brutus' body.
This was the noblest Roman of them all.
All the conspirators save only he
Did that they did in envy of great Caesar.
He only in general honest thought
And common good to all made one of them. (5.5)
It is because of his introverted character that Brutus is so naive about other men. He assumes that other men are like himself. But Cassius and Antony are obviously quite different. They are worldly wise. They understand that most men are selfish and greedy, like themselves. It is easy for both of them to deceive and manipulate Brutus because he seems so childishly simple and gullible to them. All they have to do is pretend they are equally noble-minded and idealistic. Perhaps these two cunning men would really like to be equally noble-minded and idealistic but are incapable of changing their characters from what they are. Cassius and Antony both admire Brutus but secretly despise him.
Cassius wanted Brutus to join the historic conspiracy because Brutus' sterling reputation as well as his distinguished ancestry would attract other important men to join them and would also make the assassination seem necessary and patriotic. Cassius, however, did not anticipate that Brutus would get carried away with his role as leader once he had committed himself. Brutus made serious mistakes after the assassination, beginning with permitting Antony to deliver his devastating funeral oration.