Brutus has been acting strangely because he has a lot on his mind that is troubling him.
Explanation:
Brutus was conflicted about the possible dictatorship he thought Caesar could begin and therefore had been acting strangely. Cassius asked him why was he acting so strange and offered him to be his mirror because he considers that Brutus did not see himself as the rest of the people did and want him to feel the respect they feel for him. He did that not because he was a good friend, he did that to start to convince Brutus that people support and respect him and to lead him against Caesar and his growing power among the Roman people.
