You are a witches familiar. The witch that you were supposed to companion with was not a good person. They killed the innocent for fun, so Sam and Dean, of course, had to stop him. And they did just that. Dean was the one to officially kill the witch, but it left you with a problem. You didn't have anyone to guide or companion with, so you attached to Dean in a desperate attempt to be someone's familiar. You were now Dean's protector whether he liked it or not.

Dean didn't like it. Not one bit. But, he knew he didn't have much of a choice. He couldn't leave you without a purpose, so he decided to take you on as his familiar until he could find a trusted witch to take you on. He was a hunter, though, so having a familiar was weird. He didn't use black magic. So, he was stuck with a being who could shape shift and was now practically his protector.