Some people lie to get something out of others or get out of responsibility (used to have a coworker like that, she lied non stop), some lie because they are in denial and just can’t face the truth (many addicts do that), some lie because they feel inadequate and to make themselves look better than they really are (lie they are in relationship when they aren’t, lie they have a degree when they don’t etc etc). Ton of reasons

I know someone who lost contact pretty much with everyone including her own children who finally gave up on trying to reason with her but it wouldn’t stop her from lying. Interesting thing is that her life isn’t getting better because of her lies, it’s complete and utter mess but she keeps on lying. She does have a personality disorder. But it doesn’t mean of course that everyone with personality disorder lies.

My brother had a coworker who even when confronted with evidence and facts wouldn’t stop lying. His lies were harmless like being related to someone or being born somewhere or having an award in something and more of the same (none true). Other than that my brother says he was a great guy. Go figure