i need to understand how a person becomes a compulsive liar?

It's sad but honestly there is no way in helping these type of people. People with OCD, in his case Obsessive Lying Disorder, tend to believe their own lies and are convinced more than others. They are perfect liars and are really good at it. They can keep a straight face and can even pass a lie detector. You may not believe them but they will put up a fight to prove themselves. They get pleasure by believing their own lies. They lie when they don't even have to lie. Little things like what did they eat for lunch. They might say something like " I ate a juicy steak" but in reality they ate McDonald's. They think what they say and say what they think without actually thinking about the question. People like that have a serious problem....because since they believe their own lies....they have trouble keeping in touch with reality and their "fantasy". Stay away from people like that. They tend to only think of themselves. I once knew someone who was a compulsive liar and when I called them out on it and told them what I said above, that person broke down into tears and asked for help because he didn't know why he couldn't stop lying. Stay away from people like that. They are unpredictable and are a disaster waiting to happen. Look out for yourself because this person may ruin your reputation, steal your stuff, and or even have the capability to steal your or someone Else's identity....because they don't even know who they are.

If they want help, they should go to a psychiatrist. But they have to want the help. The reasons for doing this are as many and as varied as the compulsive liars.

One reason might be is because they think they can always get away with lying. Another reason is because they are afraid of what might happen if they tell the truth so they lie. They might even lie so big that they themselves start to believe it.