In season 9 Gadreel tells Dean that if he tells Sam that Gadreel is possessing him, then Sam will be able to expel Gadreel. Now this is interesting because no other angel past or present seems to be afraid of being expelled if the host doesn’t want them anymore. Jimmy Novak was definitely sick of Castiel in season 4 according to what he said but he did not expel him from his body and return home. Neither did Caroline who was hosting Hannah though she wanted her husband when he came for her in season 10. But when Lucifer was possessing Sam in Season 5 and Sam failed to overpower him in the beginning – he was fighting Lucifer’s presence but he didn’t expel him. was it because none of this people knew at the time that they could just tell their parasite angel to get the fuck out or…? what happened? Of course if Sam had expelled him he might just go back to his old host and they’d be back to square one but still…Its a question.