supernatural - encuesta - What Is Up With Sam? 

You know what? I'm going to just say it, it's been in my head for days. I believe that he is possessed by something/someone (i.e. Michael) and that's why he doesn't care for Dean anymore. You know that Michael dislikes Dean very strongly for saying 'no' to him, so the pay-back is just sitting back and watching Dean turn into a vampire and such.