Shavua Tov. Today's Parsha, ויקרא, discusses the various sacrifices in the Beit Hamikdash with emphasis on שוגג, involuntary offenses. When a person violates an עבירה that carries with it כרת or death by the court, and he wasn't aware of his sin, he must offer a קרבן חטאת, a sin offering.
The idea here is that one must always strive towards getting closer to Hashem so that even a mistaken violation distanced him from G-d. The idea of the קרבן comes from the word קרוב, to get closer. The sacrifice again brings him back to where he should be.
In the case of the נשיא, who could be a head of tribe or even a king, the Torah says, אשר נשיא יחטא, WHEN a נשיא will sin and not If he will sin. Those three words first letters spell אני, which indicates selfishness and thinking that everything centers around him.
When a person is in a position of power the ego gives him a sense of entitlement that everything is coming to him. It's amazing how the Torah understood human nature.