Young artists are quite weird, particularly in middle school (11-14yrs).
Often they are very very proud of their work when showing it say, to their teacher. But they are incredibly embarrassed by it when it comes to sharing with their peers. It doesn't help that this is a common age where ones perception is much better than ones ability to execute a drawing, which makes it all the more frustrating. (this is something that osculates throughout life perception vs ability, but it's apparently at it's most frustrating for middle schoolers) And on top of all of that people in that age range have very fragile egos relative to other age groups.
Which is probably why they are quite happy to post their stuff online, while at the same time lamenting how much they hate their art. They're in this weird head space where they're proud of their work, but also very embarrassed and aware of the flaws in it.
I think many whether they are young or not are also incorrectly hoping for some constructive critique, online it's really difficult to find it though because most people don't know how to critique in a constructive way. (This is setting aside issues like "it's my style" and the artist not being able to take critique).
I don't think that disliking ones work is a good reason not to post it, it opens up the chance to get outside feedback on it. And others may like it and tell you why. Or they may also dislike it and tell you why. I see it as an opportunity to learn if it is used in that way. (granted posting something you hate and then belittling it is not the proper way to post it if you want to learn something from it.)