Likely not just one reason. Probably different reasons depending on the person. I knew one person who did it for attention. Another who did it in order to control situations via controlling people. One of my siblings did so in an effort to impress (zero self-esteem). I once had a friend who did it because she was ashamed of who she really was and the life she was living...so she created a fake life in order to avoid embarrassment or being "found out."