Government: Chaos in politics or government never works. Shakespeare uses this theme or view of government in several of his plays. Brutus joined the conspirators because he wanted to preserve the Roman Republic (their form of government). However, because the assassination created chaos and turmoil (basically anarchy for a while), Brutus's cause was lost, and Rome eventually ended up as a empire. History proves that Shakespeare is generally right about government and chaos. Most dictators who are currently in power created or took advantage of chaos in their countries to rise to power and establish totalitarian rule.
Life: Most humans seem to want to live their lives so that they will be remembered well after they die. At the beginning of the play, Brutus makes it known that he does not want to go to his grave while allowing the Roman Republic to falter (with Caesar being crowned king). His funeral speech also demonstrates that he cares a great deal about what the Roman people think of him and his motives. Likewise, Caesar wants to be remembered as brave and is willing to risk death (despite his wife's dream and the other bad omens) so that he does not look like a coward. Shakespaeare uses several characters in the play to show that most people live their lives in such a way that their physical deaths will not be the death of their memories or legacies.
Friendship: Quite simply, the play illustrates the truth of the adage, "Know who your friends are." Shakespeare's Caesar evidently trusted Brutus and viewed him as a friend, and thus, felt betrayed when Brutus turns on him. Similarly, Brutus and Cassius appear to be friends at the play's beginning and even in Act 4, the audience can see that Cassius feels slighted by Brutus in regards to the friendship, but Brutus does not seem to care.