In my case, it's not even fear of appearing incompetent. It's fear of being accused of or treated as if I'm incompetent when I'm clearly not. This past couple few decades the average population have become more and more downright vicious, cruel, and absolutely sure of themselves, even when they're utterly wrong about something they say or do, and as that problem grows, I become less able / willing to open myself up to that sort of potential abuse. I've got things rough enough already without that additional stress.

In addition, there's what a previous commenter said about knowing that others also have their own things they're dealing with, and I kinda don't wanna be the guy who adds to that any if I can avoid it (just in case they're not that other type of person I mention here).