Awe, this is interesting. I need to rewatch that scene, tbqh, because in the moment I don’t remember thinking Sam was sad. But all the screen caps and gifs sure do make it seem like he was, hahaha. 

But I like your reading of it, because I really love Sam and Castiel’s friendship and it’s clear that just in 10.17 they got more done in a day and night than Sam and Dean have been able to do in months, hahah. I think it is because Sam will listen and trust Sam in a way that Dean will not, so things go smoother and in turn this makes Sam feel more confident in himself because he’s not being questioned at every turn or watched because his partner is constantly worried he’ll get in the way. 

I think there’s a lot to be said about the Samsel in Distress trope that has started in the last few years. It’s something that makes a lot of Sam fans angry (myself included a lot of the time), but I also think there’s an in-universe explanation for it. When there’s a relationship like that of the Winchesters’, where one is constantly expressing doubt and criticism of the other, even in the most subtle of ways, it’s not odd for the other to start doubting themselves too, which would then affect his performance in the field. It’s kind of like when you may be totally capable to live on your own and be an adult, yet as soon as your parents visit, you spill the tea you planned to serve and burn the main course at dinner. 

We are absolutely Sam-positive! We even had an F-Yeah Sam panel last year. (No one has submitted something similar yet, but that could change any day, or your sister could submit it herself!)