Mary Sues are often compilations of everything the creator finds interesting. A lot of the time these characters are made by younger people. In the point of life children will be going through a lot more than they are used to and seeing the world isn't as colourful anymore. Making characters could be an escape from the stress of reality. Another reason is because people look up to certain characters. Mary Sues used as self inserts to video games, TV shows or movies would be made to be cooler than the original cast and admired by the artists favourite characters. Mary Sues could also be made just because the artist thinks they look cool. (e.g. Edgy Sues and SparkleDogz) Another possibility is the creator doesn't realize that they have made a Mary Sue. They may just forget to give a character flaws because they want their character to appear in a good light. It is completely okay to have a Mary Sue but it also imposes a problematic standard by having zero bad qualities.
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How to: MARY SUE
RandomA guide to the chaotic world of Mary Sues. What's wrong with them? How do you recognise one? Why did people make them?