Why Lie?

Why do so many people lie about so much?
Why is it so hard to stop making things up? How can these people live with themselves when they spend their days telling stories? How can they go through life as someone else? Someone they've made up?
I personally don't tell the truth one hundred percent of the time, but I try to as much as I can. But, being discreet and outright lying are very different.
The worst part is that so many people claim that they never lie. This is not only virtually impossible, but also kind of ironic - lying about telling the truth.
I just don't understand why so many prefer to live life stuck in their webs of lies; honesty is much less stressful.