It becomes second nature. You hardly notice that you do it. Asked why, you can give no reason. You have crossed the border between lying and being a liar. Underneath the previous stair is self-deception, for beyond a certain point, a person starts losing track of truth.
Rationalization follows next in order.
As your grasp on the truth continues to weaken, you come to blame its weakness on truth itself. It changes shape, moves around, just won't sit still. Not at all fair of it, but everything is shades of gray anyway. How silly to believe in absolutes. Truth is what we let each other get away with, that's all.