Sam Winchester is incredibly private, but also? The show follows Dean's perspective to the extent we will watch him sneak behind a building to privately punch out a wall or collapse in tears. Even when it's implied well that he's hurting, we as an audience follow simply to witness how deep the wound goes. We see him dorky by himself for no reason. On Winchester downtime, we see him hang with Castiel, Krissy, Claire, Crowley, Charlie, Bobby, Jo, Ellen, much more than Sam. Dean can be funny, heartbreaking, enraged, cruel, and so very vulnerable on the regular as Mr. Tough Guy may recommend bottling emotions... but he sure keeps that bottle on his sleeve.
Unless plot-relevant, similar Sam moments are hidden from us and suggested through the times he ultimately shatters --or, delightfully, the rare instance an episode calls for a Moose detour. Just Your Imagination, anyone? Marshmallow nachos? I mean, what. And yeah, so very lonely, not only excluded from the world, but the third wheel in his very own family. If we take that a step further, ~lonely~ as an outlier of humanity, arguably a branch-off entirely into something not quite natural. The last of his ~species as a special child. To ignore Sam's soul or, uh, blood-deep difference is to miss something foundational about him as a character imo. There is a caged creature beneath his skin that he is constantly warring with himself to drown.
Sam bottles his emotions with remarkable success after their father's passing ( lbr, those two were PURE PASSION in scenes, amazing, beautiful ) and subsequent unsavory discoveries about his being. Where Dean lashes out, Mr. Totally Normal -concerned about his confirmed difference/evil/darkness- lashes inwards, and he becomes quite accustomed to taking the blame for things beyond his control. In him is a gentle brokenness. Notice how often he befriends the misunderstood or outcast, how much easier it is for him to comprehend doing wrong for a good reason and forgive even those that have done him wrong. None of this is particularly ~quirky~ ; it's actually heartbreaking and, er, a bummer? Tough for people to hop on the Sammy-train without sensing a deep ache.
I do wish we learned more of his quirks besides coulrophobia (does this count as a quirk?? poor guy) and his weirdo love for true crime. Also wanted more time with his faith. It was so pure and innocent, and the scene Sam meets Castiel? Phew. Again, more sadness drenches the fact his faith derived greatly from his need to belong to something greater and not be whatever he is. In the end, that, too, ends up tainted by his very existence. Everything about his life is just faintly tinged sad and suffocating. Everything, from his quirks to his joys to his self worth, is restrained by what he views as necessity.
Okay. I thought I was done, but apparently not. I DO want to point out the creature I mentioned, that he often drowns, is very much alive and electric. When someone(thing) threatens his family or has, in fact, harmed someone he cares for? Hoo'boy, there's a sure chance he may backslide into bloodthirst mode. He is easily the scarier Winchester when push comes to shove, mhmmm. If that's boring to someone, I dunno what to say.