My first wife was a pathological liar. I'm bad at reading people and tend to think the best of people, so I remained naive (or willfully blind) to it for a long time. It came to a head when a sweet, mutual friend of ours asked an innocent question about what we were doing later in the day, and my (now ex-) wife gave a completely made up answer. It wasn't a white lie to get out of something, or a deflection to keep privacy, it was simply a pointless falsehood clearly produced on reflex.
She was good at keeping track of who she told what lie, so this was the first time I'd been the 3rd party observer to something I could definitively know she was lying about. I confronted her about it later privately, " is our friend, why would you lie to her for no reason?" and she gave a dismissive non-answer.
That was the first loose thread that unraveled the whole sweater. It turned out she'd been lying to me and everyone close to her, about things big and small, for many years.
It's difficult for me to understand the psychology behind it, but I think it comes down to she feels alternately safer and superior when she knows some truth that someone else doesn't know. Her parents are estranged but stayed together, constantly low-key gas lighting each other. Maybe growing up in that environment is why she reflexively lies. Or maybe it's heritable and the lying leads to that kind of relationship. Either way, I wasn't going to live the same way and peaced out.