Dean had ingested a potion that would help him and Sam with solving murders. Unbeknownst to him, he is starting to exude behaviors that are dog like. Sam was starting to notice too.

“Dean.. I think you’re becoming like a dog..” Sam says carefully. Dean gives him an incredulous look. “What?”

Sam gestures to Dean. “You’re scratching your hair.. you just barked at the mailman.. you are playing fetch-“ Sam throws a piece of paper.

Dean looks at the piece of paper and he sort of makes this whining sound at Sam. He looks at the German Shepard next to them and it starts laughing at Dean. Great.. Dean is becoming a dog.