The Mary Sue: Who Are You?
One of the reasons fan fiction is seen as an inferior literary art form is the fact that fan fiction writers change what they don't like about their favorite series to meet their emotional or logical needs depending on how the original failed to satisfy those needs. Such meddling is frowned upon because it is often frivolous and self-serving. There's nothing wrong with that provided it's kept private or shared amongst the few close friends who could appreciate it. However, there is one taboo that hits many a berserk button, and that's being called out on the Mary Sue character.
According to tvtropes, there is no set or agreed-upon definition of a Mary Sue other than a general description and types that exist. A character doesn't even have to be self-inserted to qualify. It also depends on the eye of the beholder - a character that is not a Mary Sue to some people can appear to be so to others depending on their point of view. Take the main character from Fruits Basket as an example. She may not be self-inserted, but she is an outsider joining a household with a deep dark family secret that she inadvertently and inevitably gets involved in. Some people didn't like her at first because she didn't seem to have any glaring character flaws or because she just seemed to make everything better for everyone and most of the other characters liked her. Said people let her off the hook once the story progressed and she revealed some secrets of her own. Another example of canon-Sue can be found in Tokyo Mew Mew a la Mode, in which the leader is temporarily replaced with a new girl while away and the group just automatically embraces her. This title only lasted two volumes and was rushed to an end. Was it because of this character? Who knows?
The reasons fan fiction writers would have for inserting themselves as a character in their favorite series are many, but only a few are apparent and somewhat selfish. Some, on the other hand, just want to be part of the action; these are more often than not latchkey kids whose friends have different interests and so they are left to their own devices anyway. Those who want to "fix" an event or course of events by using their self-inserted character to do so are not let off kindly by those who were perfectly happy with the way things were. While some are demonized, rightly so or not, for just attempting to be the hero for vain-glory or other personal reasons, others are not always in it for themselves. Sometimes it does take an outsider (or in this case, member of the audience) to solve problems that the characters cannot or for some reason just don't get around to. If you wanted to prevent a tragedy that the characters can't because they don't have the same information the audience does, why not jump in and do something about it? It doesn't necessarily mean all the characters have to love you for it. In fact, probably the main reason why people hate Mary Sues being loved by all the characters they "save" is that the characters' personalities may have to change drastically in order to do so. Also, and this may sound a bit cruel to some, you have to admit to yourself going in that not everyone can actually be saved.
Remember that the key word in fan fiction is fan (and also fiction, taking into account that it's also a fictional account of a fictional story), as in fangirlism or fanboyism. Also keep in mind that Ariel from The Little Mermaid was also a fan girl in her own right, and all she wanted was to be "part of that world." That being said, you should not jump to conclusions when you think you've identified a Mary Sue. Even if it's something that angers you, just let the fans be fans and ignore them whenever possible. Not all of us are psychos, after all.
Source Consulted: tvtropes.org/pmwiki/pmwiki.php/Main/Mary Sue