More than lying, politicians talk in incredibly vague terms. They talk about ideals and goals rather than specific measurable objectives.
The problem comes when people interpret those ideals unrealistically (as happened with, for example, Tony Blair and probably Obama). You can see them, just as we did 11 years ago, literally dancing on the streets.
People start expecting world peace, a property boom, flawless universal healthcare and the best education system imaginable. And when, several years down the line, life isn't perfect and the leader didn't turn out to be a magic wand, they feel misled and let down and angry.
If everyone took a deep breath and a reality check from the off, maybe they'd avoid ending up so needlessly disappointed.