The second major thread I'd like to discuss is Sam's point of view. He is an important witness and reader of Dean's behavior and Dean's interactions with Cas, but his own attitude towards the angel says much. Let me sketch it out:
Sam and Cas:
The younger Winchester meets Castiel a bit later than Dean and Bobby. During their first encounter Sam is bouncing off the walls; he's outgoing and positively disposed.
We know that Sam is a believer. Of course he'd be excited to meet an angel of the Lord. The problem is that there are two angels in the room, but Sam is excited to meet only one of them. He emphasizes it by mentioning he's heard so much about Castiel. Moreover, he doesn't show signs of veneration or religious rapture. He reacts exactly in the way a guy would react to meeting someone his sibling or friend can't stop talking about. Sam desperately wants Cas to like him back. While introducing himself he is uneasy, but hopeful. He's petrified by Cas's reluctance to shake his hand (and you have to admit that their acquaintanceship does not start well).
In seasons 4 Cas spends much more time with Dean alone than with both brothers and there are no open, friendly conversations between the angel and the younger Winchester. Information about Castiel's death in Chuck's kitchen does not affect Sam at all. There is one situation in season 5 in which Sam asks for Cas's help; the voicemail he leaves for the angel is curt and full of awkward pauses, like it was not natural for Sam to even address Cas. During their meeting they are both tense and they get down to business immediately.
In the beginning of season 6 Winchesters face another mystery and try to ask for Castiel's help. We learn that Sam has been trying to reach the angel and got no response, which leads him to a conclusion that Cas refuses to answer his prayers because he is a selfish sonofabitch. Let's retrace his train of thought. Sam knows that Cas has been killed for helping Winchesters. Twice. Perhaps he knows that Raphael has beef against the renegade angel. He is not an idiot, so he must presume that after averting the Apocalypse, locking Michael in the Cage and spreading rumors of God's death Heaven is in disarray. He knows that angels are generally pretty brutal for their own kind; torture and murder are their methods of choice when resolving their domestic disagreements. Yeah, there's no way that Castiel is in trouble that prevents him from answering Sam's prayer. He is unresponsive because he is a sonofabitch and doesn't give a rats ass about the Winchesters. Sam's judgment is no longer a sign that he does not know Cas well. Sam dislikes the angel.
Seasons 7 and 8 are charged with repercussions of Castiel's betrayal from season 6. While the Winchesters fight Leviathans and strive to find God's Word Dean is torn apart by extreme emotions while Sam is rather distanced and cool with the whole situation. He's not thrilled by the fact that Castiel is alive; not really liking him, not really hating. Of course it's because it was Dean whom Cas hurt the most...
...by resurrecting him soulless and making him hurt people, then breaking the wall in his mind and letting him remember Lucifer's torture from the Cage. Wait, what?
I got so used to thinking that it was Dean whom Castiel wronged in season 6 that I almost forgot to stop and think about what really happened. The angel hurt Sam. He killed many angels. Eventually he screwed Crowley over. He did nothing that would personally concern Dean.
The problem is that Sam treats the betrayal as something he could have expected as a natural part of his life (and certainly something he could have expected from that damned dude that turned Dean's head). Changing allegiances, secrets, betrayals - it's no novelty to a Winchester. His reaction in seasons 7 and 8 is a prolonged version of his reaction to the fact that Castiel answered Dean's prayer and not his. A huff, a sneering remark, eventually a shrug. Cas is a sonofabitch. I've never liked him anyway. I don't know what I expected. Yeah, whatever.
When Cas finally turns to the dark side it is Sam who's nearly killed, but it is Dean whose heart is broken. He is the only one to try to plead with Cas when reasoning brings no effect. Sam and Bobby explain their point of view and when it proves ineffective, they immediately strike Cas off the list of their allies. They are sad and disappointed, but not really cut-up. Like I said: a huff, a sour grimace, a shrug. Sh*t happens. Dean is the only one to believe Castiel would refrain from obtaining unlimited power just because someone begged him not to do it.
Let's consider seasons 9 and 10. According to SPN timeline it's been six years since these two met, and Sam still does not seem to feel at ease in Cas's presence. They are practically strangers. They don't seem to like each other, which is manifested by the awkwardness in their interactions while they work to save the man they both care for from the Mark of Cain. It's the situation that forces them to talk more openly and finally break the ice. Surprisingly for both of them, they find things they have in common. I'd say that Cas finally grows on Sam - not as an ally, not as someone he can trust, but simply as a person. It means that their initial chilliness towards each other was caused by something else than a personality clash. It was the situation that prevented them from becoming friends earlier, not their natures. Perhaps it has a wee bit to do with the fact that Dean is no longer obsessed with Castiel. Sam does not feel jealous anymore, so he can finally see past the fact that Castiel stole his brother's heart and really get to know him.
I've discussed the matter because it fits a scheme that we've seen only a few more times. Usually the Winchesters are one when it comes to interacting with people, especially with friends. There are few persons that one of the brothers likes and the other dislikes and - guess what - except for Gordon and Benny (and I've already explained the ambiguity of Dean's relations with these two) it is always a lover of one of the boys that the other dislikes.
Sam about Dean and Cas:
Whenever I watch fan-made clips with captions, I always stumble upon "awkward Sam in the background". Whenever Dean interacts with Cas, Sam is left out. He isn't even trying to mix in. Quite the contrary. Even if the situation requires him to react to Castiel's behavior, Sam would always look at Dean first, as if asking what he was going to do about his angel.
Season 8 brings important changes. First of all, when Castiel reappears, the younger brother is fully focused on Dean's reaction. Sam does not have any deep feelings for the angel, but he hopes that his miraculous return will end Dean's grieving and prevent him from slowly giving in to insanity (keep it in mind that Dean kept seing Cas, he's been haunted by the memory and mad with pain). When he notices that Dean is not happy to see Cas again, Sam is disappointed, perhaps even a bit nervy. Dude, you've been blubbering for him every night. Now that he's back you should be happy, stop whining and giving me hell. Why are you nitpicking? I don't get it.
Throughout season 8 Sam is left out from Dean's interaction with Castiel, but he is no longer left out from Dean's relationship with Cas. I presume that seeing how devastated Dean was by Castiel's betrayal and alleged death finally opened Sam's eyes to how important the angel is for his brother. Sam still does not socialize with Cas, but he talks to Dean about Cas. He tries to talk to his brother about his grief, his mistrust, his broken heart.
He is confused and outraged by Dean's confession he's been praying to Castiel in 8x17. He doesn't seem to approve of his brother's inability to get over it. He still doesn't understand how vulnerable Dean is; how much Cas can hurt him. Sam doesn't get it until the final scene of 8x17, when he finally openly offers to talk to Dean about his feelings towards Cas. Of course the older Winchester is extremely withdrawn, so Sam does not get the full picture.
In 8x22 Dean and Cas have an argument in front of Sam. It's not particularly fierce; I'd rather say it's sad. The way they talk and the way Dean crams a lot - and by "a lot" I mean "a lot" - of meaning into a few curt sentences indicates that Cas and him are really close; that there are undertones and hidden agendas meant only for Cas to understand (You didn't trust me? You didnt trust Me?). There is a moment when Sam wants to join the conversation, but he chokes his words back and stares at the table in front of him, frowning in confusion. It's the moment he realizes there is just too much tension and undertones in this quarrel for it to be simply a quarrel between friends; a quarrel that would concern him. It takes him a while to process this discovery. When he is convinced that this is a private conversation and should not take place before his eyes, he grunts to remind Dean and Cas about his presence. They get the message. They both clam up.
In "Hunteri Heroici" Team Free Will members have a casual conversation while drinking beer. Dean and Cas talk about April - the reaper who killed the angel. They are obviously bantering and teasing. The whole conversation would be absolutely inconspicuous if it was not for Sam's reaction. Not only does he feel left out. He's confused, I'd even say poleaxed. No matter how hard I try, there is only one explanation for his reaction that comes to my mind. Sam knows a little about Dean's relationship with Castiel, but Dean does not keep him up to date or give any detailed account for his actions. Sam does not know if Dean is OK with Cas having sex with someone else because they have broken up (which Sam kinda witnessed) and Dean doesn't care anymore or because they have sorted it out. He even has reasons to suspect that there is a hissy temper tantrum brewing behind these smirks and eye rolls. He remembers that when Castiel mentioned having sex with April for the first time Dean was hacked off and jealous... and then Cas left. He decides to get out of the way before he gets caught in the middle of another I'm not sure if it's a marital argument, oh, God, what am I doing here, so he goes to get another round.
There are fans arguing that Sam is perfectly aware of Dean's relationship with Cas and even approves of it. I wouldn't go that far. I believe that it's Sam's utter bafflement that suggests the nature and importance of that relationship. Sam knows his brother through and through. He knows exactly how Dean deals with friends and how he deals with women; how he mourns a friend and how he bounces back from breakups. The power of Dean's feeling for Cas is something Sam can't comprehend. Dean behaves in a way he's never behaved before. Sam respects it. He feels for his brother when Cas hurts him. He tries to offer help, but he doesn't know what's going on. It suggests that Castiel is not Dean's friend, because Sam would know how to console Dean after a friend's death or dissuade him from trusting a friend. Neither is he Dean's hookup, because Sam would know there's no need to offer solace after loosing a random lover. Castiel is the love of Dean's life and Sam had no idea what is going on, because obviously Dean has never loved anyone the way he loves Cas.
