Because I knew, realised now that it had only begun; there would be no end to it, not only no end to the lies I would continue to have to tell merely to protect the ones I had already told, but that I would never be free of the old worn-out ones I had already used and exhausted. (3.60)
If you give a mouse a cookie, he's going to ask for a glass of milk. And then he'll ask for a napkin. And so on and so on. The moment Lucius gives a little tug on the thread of lies, the whole thing starts unraveling, since lies have been built upon lies upon lies.
I said, and I believed it (I know I believed it because I have said it a thousand times since and I still believe it and I hope to say it a thousand times more in my life and I defy anyone to say I will not believe it) <em>I will never lie again. It's too much trouble. It's too much like trying to prop a feather upright in a saucer of sand. There's never any end to it. You never get any rest. You're never finished. You never even use up the sand so that you can quit trying.</em> (3.39)
Sheesh, tell us how you really feel, Lucius. Here, our hero inserts his adult promise never to lie again, especially after what he learned from his adventures. Lying requires a lot of effort and energy. You have to keep remembering everything you've said. And once you've told one lie, you usually have to start telling other lies, just to keep your story consistent. It's too much effort.
I didn't know what to do now; I had already told more lies than I had believed myself capable of inventing, and had had them believed or at least accepted with a consistency which had left me spellbound if not already appalled. (3.50)
Lucius feels way out of character when he lies. Is he really capable of saying such things, he wonders? Even he's unsure. But lying's not going to help him discover himself—the only way to do that is to be honest, both with others and with yourself. As they say, the truth will set you free.
And so it would be: the whole thing no more than a dream from which I could wake tomorrow, perhaps now, the next moment, and be safe, saved. So I closed the door and locked the padlock and opened the lot gate for Boon to drive out and closed that too and got in, the car already in motion—if in fact it had ever completely stopped. (3.70).
Lucius can't take back his actions. Similarly, he can't take back his lies. Once he goes through with it, there's no turning back—unless you actually just up and admit you were lying about everything, which isn't a bad idea. But it isn't any easy thing to do, either.