Which motive is most likely behind Cassius comment to Brutus that every one doth wish you had but that opinion of yourself which every noble Roman bears of you

Cassius is trying to build Brutus up here. He knows that Brutus is humble and that everyone respects and loves him, but he wants to reinforce that fact to Brutus so he is willing to trust all of the conspirators that Cassius brought with him to Brutus' house. He is also trying to sneak in a sense that everyone wishes Brutus would understand that he's just as good (if not better) than Caesar.