First of all, whenever you post a story for the WORLD to read, you open your work up to criticism. That's the flip side of free speech: if the Suethor can write whatever she wants in her "very own story", then we have the right to say whatever we wish about it.
This, of course, begs the question: if you are so attached to your precious story that you defend it at the price of good writing, why in the world are you posting publicly? You have just done the equivalent of going on stage naked in front of a crowd whose objective is to assess the merit of your body. It is not a prospect for the highly sensitive.
Suethors take this a step further. Most of them put little effort into their stories. They can't even bother to make up a new character... they just recycle what they know of themselves and spruce it up a little! Then they demonstrate love for neither the canon universe, nor writing itself, but only for their beautified self-insertions.
Not only do they put up poor work in a place where it is vulnerable to criticism... they then beg, B-E-G in big, bold, underlined, misspelled letters, to PLZ REED N REVUE!!!!! (I'm not sure if I have ever seen a Mary Sue fanfic without this.)
That is like going up on a stage naked, not having bathed in a week, with un-brushed hair, chicken pox, and a thorough drenching in sour milk... then screaming out loud to the observers, "TELL ME WHAT YOU THINK OF ME!"
... Why are we mean for saying that it stinks?