Clinton lies even when he doesn't have to. No one ever explained to him casual lying ultimately gets you in trouble. That's why its like a comfortable leather shoe that's too hard to give up to that man.
Beyond that, he even lies when he's right.
I remember back when he was first running for president in '91, the National Review editorialized that their problem with Clinton was not that he lied, which all politicians will do sometimes, but that he lied even when the truth would do him more good. That turned out to be one of the most prescient comments ever published.