Commitment and Consistency
Once you say it out loud, your brain will lie to keep you consistent with it.
Grounded in the research on Robert Cialdini — Influence: Science and Practice (Consistency principle); Foot-in-the-door technique (Wikipedia)
What is commitment and consistency?
One of Robert Cialdini's six principles of persuasion. People feel an almost obsessive pull to act consistently with what they've already said, done, or written down. Once you take a stance, you're under pressure (from yourself and from everyone watching) to behave in ways that justify that earlier stance. Cialdini's line: people live up to what they have publicly committed to. The commitment is the hook. Consistency is the leash.
What's the actual engine under it?
Cognitive dissonance, from Leon Festinger (1957). Holding 'I committed to X' and 'I'm not doing X' feels physically wrong, so your brain resolves the tension by changing the easier variable: your behavior and beliefs, to match the commitment. Backed by Daryl Bem's self-perception theory too: after you act, you look at your own behavior and decide 'I guess that's who I am,' then keep proving it. You're not staying consistent for them. You're doing it to stop your own head from itching.