This in 12x16 felt like another solid character moment, to me; not just the fact that Dean left, but that Sam stayed. I think it’s the same difference in attitude that prompts Dean to squash down and deny the things that worry him (or, the flipside of the same coin, to accept them with fatalistic helplessness) whereas Sam will stare steely-eyed and insistent into the face of his problems. (Of course, that can certainly be problematic too.)
I saw one of your tags say that you're not sure Sam would be ok with Destiel and I'm curious why. Do you think it'd be like the Benny situation again or something else? I didn't ship Denny but I don't think Sam likes when someone else comes before him in Dean's eyes. (I can't see Sam not liking Desitle as a homophobic thing, but ya never know.) Anyway I'm genuinely curious because I could see Sam not exactly like a shipper but ultimately ok with it.