I mean, I uploaded some posts recently and they were rejected and most of them were normal, but I can't know the reason, therefore I can't know how to avoid that in the future, and the other boorus tell you the reason. Your post was rejected. Isn't there some way to know?

That is honestly good question, as of now there's no way of knowing who or why ones posts get denied and it would be such a help if they at least gave a short message like "Post deleted for following reason..."
I have tried doing some self uploads and it is so damn disheartening seeing one after the other get deleted even though I try my best to make em high quality and tagged properly.

Yes that would be easier or help you in the future or at least get an automatic mail form moderation.