Humility and Denial
In the norman's mind everybody who is good at something needs to live in denial about being good at that thing. They have a completely twisted idea of humility and arrogance wherein they think everybody who says he is good at something is just an arrogant asshole and probably is not even as good as he claims. Children are being taught to live in denial about their own skills and talents and are being manipulated into this fake humility. They try to make it relative to make it look absurd – as a cheap rhetorical trick – and say things like "I'm not the best in the world", or "There are better people than me" completely disregarding that being good at something doesn't mean to be the world's best. This tendency for having an inferiority complex that Kaczynski noted leftists to have is a thing that almost all oversocialized people have. Even if they are the best person they know in some area they will still claim that they aren't very good. To a degree this is also performantive; a lot of those people probably are self-aware about their abilities and will deny them regardless in fear of being seen as arrogant.
This kind of thinking does great harm to your mind, because this is the perfect starting point for depression and self-hatred.
Some even have this deranged concept of "The better you are, the more you are aware how bad you actually are". This whole """Dunning-Kruger""" phenomenon (Redditor phrase) only makes sense if you don't account for the last third of the curve where one actually gets good. To them you either have an unrealistic self-image and only consider yourself good, because you are a bad at it, and cannot see the horizon, so to say, to consider the things you don't know, or you are in the middle, where you are aware of the many things you don't know yet. They never mention the last third of the curve where you have a realistic idea what you know and don't know and still are good at it.
Tangentially related: A friend of mine once noted that the normalfag's concept of self-awareness is self-flaggelation and it is very true. To them you are self-aware if you say how much you suck at everything and how worthless you are, because you can only be aware about your flaws, not about your talents or skills. This ties back to the general notion of expecting people who are good at something to have a huge inferiority complex.