more thoughts on "who would dean have a harder time living without, sam or cas?" that i didn't want to dump on the poll
there are two scenarios here: 1) they're dead, or 2) they're alive and dean knows they're doing fine, but they're not in his life.
in the first scenario, the answer is objectively and provably cas. sam died and dean did his best to live a peaceful life with ben and lisa, and mostly succeeded. cas dies and dean kills himself the first chance he gets.
it’s not a matter of who he cares about more! he cares about them differently, it's not something you can compare like that. also, dean is capable of surviving the deaths of people he loves very dearly despite enormous grief (mary, jack, bobby, charlie, sam in s6 etc.). the deaths he doesn’t survive are sam in s2 (suicide via demon deal), cas in s12 (suicide via syringe), and cas in the finale (suicide via rebar, though you can argue this one).
the reason dean trades his life for sam’s in s2 isn’t because he can’t live without sam, it’s because he still believes that his entire purpose in life is to protect sam, and that he’s worth nothing if he’s failed that, with bonus guilt on top that john already died for him. the grief is a contributing factor but the guilt of failure is what kills him. and i don’t think he would have killed himself if it wouldn’t have gotten sam back. because when sam dies in s6 dean struggles and is depressed and grieving but he's trying, and it’s still the best year of lisa’s life. and that was with cas gone and while also trying to shove a lifetime of ptsd under the rug!
when cas dies in s7 dean is depressed and passively suicidal to a degree that terrifies sam and bobby. when cas dies in s12 dean kills himself 5 episodes later. and when cas dies at the end of s15 dean is dead within six months, in a way that very much reads as at least passive suicide. when cas is dead, dean does not want to live. and we are shown several times that despite his best efforts, sam is not capable of picking up the pieces and getting dean through that loss.
in the second scenario, alive but apart, imo the answer is also cas. at least by the end of the show.
sam is the little brother dean raised. he loves him, he'd miss him like hell if they couldn't ever talk or see each other again, but ultimately dean's happiest ending has always been one where sam is living an independent, happy, fulfilling and (relatively) safe life. it would just majorly suck if they couldn't be a part of each other's lives at all. your kids grow up, yk?
cas on the other hand is, by the end of the show, dean's chosen life partner. by the time romantic confessions happen, cas and dean have already been filling the role of spouse for each other for actual years. cas is the person dean leans on for support, both practical and emotional. cas is the person who sees him clearly and who he chooses to open up to. dean is far, far more resilient when cas is around! not having cas in his life means not just losing the life they had and the future dean wanted to build together, but also losing his emotional rock and the person he, a chronic caretaker in most of his relationships, is most comfortable relying on for support. so in that sense I think life without cas would be harder than life without sam.

















