People lie on the net for the same reason they do in their real lives. Because they think (or simply because it's true) that they have a boring existence and the only way to get attention from others is through lying.
People paying attention to you makes you feel better, especially if you have low self-esteem - which the majority of liars do have.
Same on the net, they lie to get attention simply because they can't get it in their real life. Some of them are afraid to lie big-time in real life because they are too shy or it simply because no one would believe them ("I am a tall curvy blond" example when they are short fat with a moustache). It allows them to set their own rules and attract others for their few minutes of them. Then simply change username (or in real life find a new friend) and repeat..and on and on and on..
That's life.