The reasons politicians lie is because the public doesn't want to hear the truth. People want to hear what they want to hear. When two candidates are running and one of the tells the truth and the other says what the public wants to hear, the one who says what the public wants to hear wins the election. Thus, and there are exceptions to this, if you want to win an election, you better start lying, because the guy who's telling you the truth doesn't have a chance.