It is precisely because Brutus is an individual beyond reproach who sincerely loves Julius Caesar that Cassius so avidly struggles to recruit the former to his cause. In Shakespeare's play, Cassius is but one of many members of the Roman elite who comes to fear Caesar's growing power and megolamania. Caesar's popularity with the Roman public is viewed by members of the Senate and others as a potential threat to the Republic and to their own good positions in Roman society. Brutus loves Caesar very deeply, but is among those who fear that Caesar is accumulating too much power and is veering towards autocracy. In Act I, Scene II of Julius Caesar, Cassius and Brutus have a protracted discussion of this matter. Cassius, the more cynical and manipulative of the two, appeals to Brutus' sense of honor and to the latter's commitment to the preservation of the Republic:
I have heard,
If Cassius is confident that he may just have a kindred spirit in Brutus, at least within the narrow parameters of their mutual fear of Caesar's growing power, Brutus's response to the cries of adoration he hears from Rome's streets further cements their common bond:
So, we know that Brutus, despite his affection for Caesar, is very worried about his friend's threat to Republicanism, and Brutus is very pro-Republic. Cassius needs Brutus to join in the conspiracy against Caesar because of Brutus' commitment to the Republic and, more importantly, because of Brutus' love for Caesar. Such an alliance of conspirators will provide the political imprimatur Cassius wants to undertake so draconian a measure as assassinating the leader of Rome. And, Cassius is not alone in this thought. In the Scene III, Cinna, a respected poet and participant in the conspiracy, says to Cassius:
Yes, you are.
O Cassius, if you could
But win the noble Brutus to our party--
Brutus's is the linchpin in the conspiracy. That Shakespeare quotes Caesar at his moment of death as uttering the famous line "Et tu, Brute. Then fall, Caesar," is testament to Brutus' importance to the success of the conspiracy. That one so close to Caesar could join in inflicting mortal wounds on the fallen leader lends his demise a extra poignancy.