One reason is fear. When children lie out of fear they do it because they know that what they did was wrong and they're afraid of the consequences. Another reason why kids lie is
that it's a habit. This means that they've lied so
much that it's an instant reaction. They'll lie about dumb little things that don't matter as much then, but later on they'd have to keep lying just to protect the first lie. Modelling is another cause
of lying. This is when the child is surrounded by people who lie constantly or when adults have made it seem as if lying isn't such a big deal. Last reason is due to over prediction. When they lie because of over prediction, they do it thinking that their parents will over react.
This image shows two people talking business. One of them says that in that situation he suggests they lie, meaning they know they did something wrong so they'd prefer to cover it up. There have been times when I was younger when I'd lie to protect what I know I did wrong. I did it because I was afraid of my mom grounding me or yelling at me. I later grew out of it, well kind of. I still do it once in a blue moon just to keep her from nagging.
This cartoon shows a man who's lying because of habit. He's telling his partner that whatever he just said he made up right then and there, probably out of reflex.
I've also lied because of this. I got used to lying so much that every time I got questioned about my whereabouts I'd lie, even if I wasn't doing anything wrong. This was a stressful thing to do because after the first lie, I'd have to keep lying over and over again just to cover up the lie I told first. I found that making lying a habit was too much work for me so I always thought twice before answering to my parents with a lie. I still say a couple of white lies to them from time to time though.