When Sam knew that Dean was an omega, it was six months after Jessica’s death.
Honestly, he was very surprised, as in what-the-fucking-hell-is-going-on surprised. After dozen times of sniffing and nuzzling and snuggling and spooning – which made Dean pissed as hell – he was certain that it’s not a witchery prank or demonic omen.
And when Dean couldn’t stand with his brother nuzzled his neck like a dog more than he liked and asked him what the hell was wrong with him, the first thing that crossed in Sam's mind was…
“Dude, you’re an omega?”
The blank look on Dean’s face made him felt like a stupidly idiotic moron.
“What kind of stupid question was that?” Dean asked him as he cocked his eyebrow. “Of course, I’m an omega.”
Sam stared agape at him.
Dean saw shocked look on his brother’s face. “You… you don’t know?” Sam could read the incredulity coming off of him. “I am an omega since… I don’t know… forever.” Dean laughed like there’s no tomorrow. “How come you didn’t know?! You were in my room when I went into my first heat, and it was about… five years ago? I remember you're doing research and slept like the dead in there when that happened. You also…”
As Dean was going on and on about what was his brother doing in his first heat, Sam felt like to bang his head at the wall.
Dean was indeed reek and smelled gross at those days, but Sam thought it was because Dean just came back from a hunt or PE in his school.
Yep, official, he was a stupidly idiotic moron.