I don't understand why some readers/commenter do this

Just a venting post but I really don't understand when readers bash on works in the comments on fics that make them angry/offended or on fics they that are just not written the way they expect. I know this by no means applies to every reader - many and most readers and commenters are actually really nice and excitable.

I can understand when you've devoted your time to a fic you think youve found a fic that fits everything you're looking for to read but then one thing happens and it makes that one reader heated enough to tear you to shreds in the comments... Like, this is fanfiction. If you don't like it, move on and find something else. And if you really can't find what you're looking for in a fic, then maybe that will inspire you to write it. And writers write fanfic for free, readers read that fanfic for free. This is all for fun and no profit. I do what I do because I love to entertain/express myself and as long as I'm not hurting anyone, I am going to continue to write.