There are some fundamental reasons why teens may lie compulsively:
Fear: Fearing the consequences

Habit: Often strengthened by hostile confrontation.

Modeling: Lying is a commonplace behavior; teens are subject lying. Moderate lying is thought of by many parents as harmless (such as a "white" lie, or a "harmless excuse") or mistakes (such as an unkept promise), or purposeful distortions of the truth ("I had to lie because...").

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Once trust is damaged by lying, it is up to the teen to prove their trustworthiness. What do you do about fear-motivated lies? Teens who lie out of fear usually know that they have done something wrong. Consider that the teen's problem is not in knowing what is wrong, but in resisting its temptation. Allow room for negotiation, compromise, listening before accusing, and keeping your volume down. Get beyond the lie and address the behavior that "necessitated" the lie. Make telling the truth a priority both in instruction and by example.

Above all, recognize that the purpose and desire of every parent is to encourage honesty. Start by rewarding positive and honest behavior. Praise your teen when they provide smaller truths. Most importantly, model honesty in everything you do or say.