I hate myself so I'm bound to hate thing associated with it, I try to hate my creations and works less by time so it will not halt my productivity.
I think it's conditioned by lack of encouragement and acknowledgement I got growing up. It is strengthened by some notion that you have to be humble or else you're the worst arrogant jerk and you're only good at thing when you're the best on it.
It's hard to dismantle the thought that having confidence in your work means you are stuck up bastard that think too highly of yourself that you think your own work is good when nothing in this world is perfect. Is it also hard to believe that you don't need to be the best on something to genuinely be that thing.
The second, when you already admit your work sucks people would be more understanding and less likely to call you out on it. Having told mistake I have made is like repeatedly being shamed on it and I hate that.