I would lie as a self-protective mechanism. In my mind I DID have to lie because otherwise it would have been unbearable.
No alcoholic who's still drinking can be honest about it. And along with that, lying gets to be a habit.
The last guy I lived with was a pathological liar. I actually kicked him out finally when I discovered he'd been lying about his AGE for five years. He claimed to be two years younger than me but was actually five years older. He mostly lied about stupid stuff to make himself sound more impressive than he was. He was pretty convincing. My kids saw through it and years later shook their heads that I ever bought any of it. On some level I think I knew it was BS, but it was mostly relatively harmless stuff.
Interesting thing, though, he rarely drank, even on social occasions. Weird, huh?