Kripke told EW in 2010 the main reason he chose to leave was that he "felt like it was the right time." The CW wanted Supernatural to continue and Kripke had ended his story with season 5 finale, "Swan Song." He stepped aside, making room for someone else to tell stories in this universe, allowing him to focus on other ...

He and Dean argue about how to stop the Apocalypse, whereupon Dean tells Sam he's changing into a monster, leading to a vicious fight between the brothers. Sam is the victor and as he leaves, Dean tells him not to return  just as their father John, years ago, told Sam not to return when he left for college.