Episode 12 - Faith
This is one of those scenes where you can, now 7 seasons later, sit back and go “and that’s basically the plot of the entire show”. But when it aired, before deals were made and they knew of the horrors of Hell, this was a first real glimpse of Sam’s feelings. The show is, for the most part, from Dean’s POV and, as such, we see from the beginning how devoted he is to Sam and Sam is sort of painted as selfish and stubborn [again, because we see it from Dean’s POV, who felt abandoned and hurt when Sam left for college]. But in Faith, we finally get to see a bit in to Sam’s head. He doesn’t sleep, spending every moment researching and calling every number he can. He puts aside his anger towards his father to call him. He refuses to accept reality. Yeah, he is stubborn, but one of the things he is stubborn about is that he will not let Dean die. Later in the series, that same stubbornness will lead Sam down a darker path, but at the roots it is always this - this pure love and devotion for his brother. Every one else in his life dies, but he simply can not allow Dean to.