Have you ever had a friend just totally and completely lie to you? Perhaps you found out that they were lying about going somewhere without you, or maybe they even lied about how they felt about something that was important to you. Lying sometimes accompanies other things like cheating, but not always. Sometimes a friend lies because they think it's better than hurting your feelings.
People don't always know how to act. In an ideal world, you'd want friends around who would be honest with you no matter what. But do you always want your friends to tell you when they don't like what you have on? Or when you aren't talented? (I wonder this sometimes about the contestants on American Idol. When they are just horrible, I think, didn't their friends tell them they weren't any good?)
Here's more about why friends lie and what you can do about it.