Why do people feel the need to make Sonadow stories

That depends on how you're asking.
If you mean by "Why do people like Sonadow?", I'm a sonadow fan so I can answer that. I find Sonadow cute, and I like it when Shadow is the dominant male in the relationship and if one of them were a girl, I'd say Sonic.
If you mean by "I know they like it, but why do they have to write about it?", we sonadow fans like to express ourselves. We like to write stories about the couples we love because we enjoy writing about our two favorite hedgies.
I hope this answered your question.