Our reasons for denying something, appear to be almost endless. Denial may be a response to negative emotions (eg, guilt & shame, regret & remorse), and/but also to positive emotions (eg, confident humility, love). From a rational point of view, denial seems illogical. So, why do we deny?
Denying is part of lying (ie, what). Children start lying at c.3 years and more at 4-6 years (source, when). Hence, the how is due to (human) nurture (eg, upbringing), and not a result of nature (eg, DNA). Some cultures have a higher or lower degree of lying. Still, we all lie and deny (source, who).
Essentially, denying – and lying – are about our handling of the truth. The film A Few Good Men (1992, IMDb) contains a famous phrase: You can’t handle the truth! (clip). In my view, it explains the Why: we assume and expect that others cannot handle the Truth. They assume and expect the same from us.
Hence, the new question is: why do we assume and/or expect that we cannot handle the truth?
A rephrased question might thus be: why is the truth never pleasant?
I suppose this is due to our (inflated) assumptions and/or expectations about our reality – a.k.a. our perception of reality. In other words, our perception of reality will (often) be more pleasant than reality. Actually, a 1968 Star Trek episode was already entitled: Is There in Truth No Beauty? (Wiki).
The solution to the above is rather simple: to manage our perceptions (eg, of reality). That should stop the truth from hurting us too much. However, the phrases bitter, brutal, harsh, naked, ugly truth suggest that we are unable to handle the truth. We lost that ability during childhood.
“When I was a child, I talked like a child, I thought like a child, I reasoned like a child. When I became a man, I put the ways of childhood behind me. For now we see only a reflection as in a mirror; then we shall see face to face. Now I know in part; then I shall know fully, even as I am fully known.”1 Corinthians 13, verse 11 and verse 12
Note: all markings (bold, italic, underlining) by LO unless in quotes or stated otherwise.