because they are afraid... they are either afraid to hurt the person they are lying to, or afraid to hurt themselves... They don't realize, that the truth always come. If you are in a relationship and you lie, then the pain in 1,2,5,10 years will be enormous in comparison to if you told the truth on time. I can forgive something if told on time, but man can never forgive and forget something, that was kept in secret for many years... This is why we should always tell the truth, no matter what.