What has strained the relationship between Brutus and Cassius

The relationship began to fall apart near the end of the play because Brutus' character had changed significantly, from a noble and quiet character to a power hungry man. And Cassius did not like this very much because it was his idea to kill Caesar so that Cassius gets the power and attention. Also Brutus blames himself for Portia's death and Caesar's death when in reality it was Cassius who manipulated him in doing it.