Many people wonder about Dark Sonic, and exactly how he transforms into what he is. There are three ways Sonic could become his dark self, and they are explored here.
Theory One: The only "canon" time that Dark Sonic was seen was in the anime Sonic X. Now, we all agree that he transformed because he saw Chris and Cosmo suffer. But remember, they were two people. Another element that could have assisted in the transformation: the thousands of fake Chaos Emeralds around Sonic that he had encountered. Their massive-yet-finite energy could have helped to destabilize Sonic, allowing his intense rush of anger to push him over the edge.
Theory Two: (those who dislike Sonamy, please don't flame this). It is generally agreed, although with some dispute, that Sonic really does love Amy Rose, but he just doesn't have the courage to admit his love. It is reasonable to assume that Amy's death, especially at the hands of Eggman, would trigger the dark transformation. If Sonic was wielding the Chaos Emeralds during such an event, he might absorb their negative energy and be an even more powerful Dark Sonic!
Theory Three: Now, this theory is a bit of a long shot. It is (possibly?) possible that something really, really enraging, like a kiss from Eggman, might enrage or gross Sonic out enough that he would transform.
Well, that's about it. Hope you liked my first article! :)