Honour is dearer to Brutus than life itself. And that is what Cassius reiterates, with the goal of attracting Brutus to his own cause (of betraying and getting rid of Caesar). Cassius masterfully manipulates Brutus. First, he tells him that he is honourable. And then, he holds him by that honour, because honourable people should act that way. Furthermore, he tells Brutus that the Romans would be eager to have someone like that as their leader. So, Cassius first feeds Brutus's ego, and then starts provoking his greed.
