Sam could see everything.
At first it had made him uncomfortable to see Dean look at a man that way. Alright, technically Castiel wasn't a man, but Sam's human brain relied on senses that told him that that was what the angel was. There had been at least a half-dozen times early on when he had seen the two of them just doing their weird gaze thing, inches away from each other, angst levels through the roof, occasionally even heavy breathing, and expected one to pounce forward and lock lips with the other in some kind of passionate, earth-shattering embrace.
It never happened, and Sam no longer expected such a thing. Castiel was an angel, most probably incapable of understanding the romantic love he held for Dean. He had his duties to Heaven, and an eternal life that Dean did not have. And Dean, well, he was just stubborn. If Sam had told Dean what he knew, Dean would have punched him out for suggesting he had feelings for a male, especially a friend. That didn't stop it being true, though. Dean loved Cas, and he was too busy crushing it inside himself with sharp and demeaning words to the poor guy that he didn't even know.
Sam wanted happiness for his brother. He wanted Castiel to grab ahold of the humanity within him that no other angel seemed to have and realize what he really felt. But that was just about as likely as Dean finally pulling his head out and building enough self-esteem to know he deserved what could happen between them.
Little glances of worry. Momentary fond smiles at each other's words. Tiny movements to close space between them. It was simple body language. And everytime Sam saw it, he lowered his eyes, heart breaking for his brother.
Sam could see everything; everything that would never happen.