Truthfulness (where truth is your attempt at accurate interpretations of your perceptions) is the precondition for thoughtfulness (where thought is your ability to synthesize, abstract, and compare).
A person (like me) may sacrifice truthfulness and their ability to think in order to gain a simple pleasure, or escape a simple pain.
The utility of a lie— to another— may be debated on the grounds of the benefits it buys compared to the (price * probability) of its discovery. In this case, the liar impairs the other’s ability to think. The morality of that decision must be interpreted in context.
In stark contrast, the lie to oneself can never provide a greater benefit than its immediate price. The self-lie purchases no more than permanent ignorance, and no less than a deferment of the shock that would accompany some understanding.
The unpleasant feelings that accompany a nasty realization can shake and disrupt someone’s life. However, that disruption shifts the being towards the ‘real.’ The real is a Good here because if you aren’t believing in some aspect of life, your brain can’t use that aspect to analyze the world around you. That ‘payment’ compounds as the perspective you’d gain understanding the event fails to inform your view on affected elements of life. You’re left with a vulnerable, dark spot in your weltanschauung, and whenever something approaches it, you either have to go back and pay the price, or you have to let the darkness expand to cover the new troublesome idea. In that way you’re led to understand less and less of life, and the price you’d have to pay to go back and understand things becomes greater and greater.
I think I’m especially prone to self deception, and huge swaths of how I see the world came into being to help me avoid understanding. For example, I was party to my mother’s immature and abusive behavior as a preteen and in my early adolescence. Suddenly I was in a position to judge the unassailable goddess of my childhood. But for a reason I call weakness, I didn’t. There were no role models for me to imitate, and I didn’t innovate. Instead I suffocated myself in fantasy novels, masturbation, and eventually pot.
The price for that comfort is your ability to think. And as a human being, that is your only really important skill. As you lose your ability to think, you’re reduced to an animal, without claws, with ineffective teeth, in a world you can’t hope to understand.