You were born a shapeshifter, doomed to never be accepted by society. You're different. Something else. You grace a different face so often you don't know who you are. You couldn't change what you were in the figurative sense of things. You were a shapeshifter bound to no one form. You had no self. You can only watch on as the world around you lives and moves on. People have lives, careers, families. Yet you don't. You wanted family and friends and to learn things like other people did, life was a privilege you were not granted, and that hurt a lot.
'Things' like you are hunted, by people like a one Dean Winchester.
Dean was on a hunt by himself, why? Because he and Sam had an argument and they both needed time away to cool down.
Dean had heard about some weird shit going down. This one town had a number of reported 'doubles' and 'puddles of flesh and gooey, biological substances' that had been discovered in sewers. These, thought Dean, are signs of a shapeshifter. But one that wasn't killing or committing crimes. Dean took it upon himself to hunt it down. It would be a nice distraction to gank whatever it was.
'It', being you.
Dean has had a little personal vendetta with shapeshifters ever since one pretended to be him and framed him for murder. That left a bad taste in his mouth.
After what totalled up to days of research and investigating as well as seducing the occasional fling. Dean finally tracked you down and cornered you, a silver knife in hand as silver was how you killed shapeshifters.
"Don't move!" He yelled at you, you just cowered in fear. Shaking and almost in tears. you didn't want to die. This reaction confused Dean, it confused him a lot. You weren't like the shapeshifters he'd had the displeasure of encountering before.
Dean's expression softened and his demeanour overall became less threatening, although not completely letting down his guard, but somehow, for some reason, he recognised that you weren't evil. "Uhh... Are you alright..?"