I agree completely with the the above post and would like to add a few things. Brutus is a noble, honorable, and loyal man. He loves his Rome and loves the people of Rome. However, he also loves Caesar. Caesar and Brutus are presumed to be very close at the opening of the play, one month prior to the assassination of Caesar.
Brutus, one of our two tragic heroes in this tragedy, is brought to ruin by his own honor and loyalty. While the other conspirators are motivated by hatred, jealousy, and greed, Brutus is motivated by the best interest of his countrymen. Cassius takes advantage of this patriotism by justifying Brutus' fears with his own. Brutus tells Cassius to be patient with his decision-making because of the special place the two parties (Caesar and the Commoners) hold in his heart.
Brutus must decide how to act in response to the presumed offering of a crown of rulership to Caesar. The Roman Senate was a democratic body of government, and Brutus and others are afraid that Caesar will be awarded special powers of solo rulership. He doesn't think that this is the correct form of government for Rome.
So, his predicament is in how to respond. Should he allow the process to take its course and, if the people seem to want Caesar (a man that Brutus does admire) as king, allow it to move forward; OR does he take sides against this potential change, siding with the Conspirators, murdering Caesar to keep Rome a democracy ruled by the Senate? Assassination is an act of treason however, so, ironically, Brutus must decide whether to act against Roman law in order to do what he thinks is best for Rome or obey the law and allow Rome to become the sort of government he thinks is wrong.
Brutus, unlike Cassius, is a cautious man. In Act I, scene ii, when Cassius all but lays his cards on the table, the only indication that Brutus gives of his discontent with the state of affairs is the following:
Be not deceived. If I have veil'd my look,
I turn the trouble of my countenance
Merely upon myself. Vexed I am
Of late with passions of some difference,
Conceptions only proper to myself,
Which give some soil perhaps to my behaviours.
. . .poor Brutus, with himself at war,
Forgets the shows of love to other men.
It will be in the later Acts of the play that Brutus' real feelings and the strain of his decision to murder Caesar is demonstrated to the audience.