One reason is that it fills a gap. Fans are always curious about the romantic aspects of their favorite characters. Sonic and Amy have a lot of potential as a couple. Their dynamic, with Sonic being a bit oblivious to Amy's love at times and Amy being so head - over - heels, is interesting. Fanfiction allows for a lot of different interpretations of how their love could grow. It can range from cute and fluffy stories to more serious, angsty ones where they face challenges in their relationship. And this variety attracts a wide range of readers.