Some people to make themselves look more important. By taking about achievements they haven't done those people hope to seem more successful than they truly are.
Some people lie because they are unable to tolerate confrontation. A little kid can lie because he is too afraid to get punished by his parents or a man can lie because he is too afraid of his wife's response.
If a person felt inferior to others he might lie to feel good about himself. A man who has no job might lie about it when he discovers that all the people he just met have successful careers.
Attention seekers go lengths to get people's attention. Some attention seekers start lying to get more attention from the people they care about (See why some people seek attention).
People can lie when their self esteem gets threatened. A person can deny doing an action that might make him look stupid or clumsy.
People can lie to complement others or to avoid hurting them. A typical compliment would be telling someone that they look good even if they don't.
While inferiority is temporary and usually felt in certain situations an inferiority complex is a more long term condition where a person always feels that he is not as worthy as others. People who suffer from an inferiority complex might lie more often to feel better.
People can lie to avoid embarrassment. If someone didn't answer the call of a friend on intention then suddenly met him he might claim that the phone was silent or that he didn't hear the phone ringing.
A person who is late might tell his friends that he is 5 minutes away while he is still at home. In such a case lying has the purpose of helping that person avoid the anger of his friends (See why we become angry).
A person can lie to himself and even believe in the lie to achieve an important goal. For example a person who bought something that he doesn't really like might convince himself that he likes it and even believe in the lie.
Men were found to lie more often than women. Some men lie to impress women. Men were found to lie the most about their earnings and careers (See the effect of peer pressure on people).
When a child sees an adult lying to evade punishment or to achieve any other goal then the child might learn that lying is the best strategy to solve some problems. Lies can be as simple as promising candy to the little kid and not bringing it.
A sales man can lie to a customer to close a sale. People can lie to achieve career related goals or to achieve certain financial rewards. A person can lie about the product he is selling just to get a higher price for it.
Some people lie to avoid drama. Men can lie to their spouses to avoid debates or long conversations that might be unpleasant.
People can lie to hide their vulnerabilities and weaknesses. In order to avoid being seen as weak or vulnerable some people can lie. A man might claim that he feels fine while in fact he is afraid or depressed.
Some people lie to create a certain fake identity. If a person wants to believe that he is too brave while he is not he might then start lying to support that new fake identity.
Humans are social beings. In order to maintain a good relationship with others some people lie. A person can lie to his friends to be seen as friendly and agreeable.