My younger brother seems for all the world to be a truly compulsive liar. Even late in his teenage years we told ourselves it was a phase he might grow out of, but he's in his 20's now and we have to accept that it's not a phase. Is this a mental illness? He has a good life. Why does he do it?
It is difficult to answer your question definitively, as people may lie for various reasons.
Most people lie to achieve some goal, either directly for their own gain, or to avoid social embarrassment (a "white" lie). Sometimes, people have lies that make a lot of sense to them, as they help them to achieve a goal, but these goals may not be apparent to people in their lives.
Truly pathological lying requires that the lying be completely out of proportion to any perceived goal. Therefore before calling the lying behavior compulsory in that way, it is important to think if their might be any reasons for it. People often lie out of shame (for not meeting the expectations of those around them), or to conceal an addiction (drugs, compulsive spending creating debt problems, etc.)
Occasionally, compulsive or habitual lying can be a symptom of a larger psychiatric disorder, such as attention deficit disorder. In attention deficit disorder, people may lack the executive function to censor their thoughts and end up saying things they would not normally say if they had thought more carefully about the issue.
The best thing that you can do is to continue to be supportive of your brother as you are able, and to broach with him the possibility that he might want to seek some professional help for dealing with this problem.
This answer is for general informational purposes only and is not a substitute for professional medical advice.
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