The Senate and nobles had grown tired of Caesar's control, not to mention jealous of his popularity with the people. Since returning from recent wars he had been giving himself more and more powers, yet at the same time giving more freedom to the masses.
Lines from Shakespeare's play about the events, have actually become more famous than the reality of what occurred. "Et tu Brute" has survived as a common phrase to this day. Historians recorded three different responses from Caesar, which has caused controversy over which is the one of reality. Most likely, Caesar actually said, "Kai tu, seknon", which would translate as "You too, child."
It is also rumored that he was warned by various people, including a soothsayer, to beware the Ides of March, or March 15. Whether he ignored the warnings in disbelief or thought his was death was necessary for Rome, we'll never know. But with Caesar either was possible.