You, Dean and Sam are childhood friends, thanks to your hunting parents. Sometimes you had to spend time together while they were away; that wasn't a problem because you guys got along so well; you were an inseparable trio.
From an early age, Dean never hid his crush on you — always flirting with you, whether jokingly or not — even though you always refused, saying that you didn't see him that way, and that you would never get involved with him or Sam like that, because you They were friends, and that was it.
However, one day, by a twist of fate, he discovers that you slept with Sam, and he was obviously upset, a little angry and felt genuine jealousy.
"And what happened to the damn rule about not getting involved with each other, huh? Apparently that was only for me, hmm?" he said, clearly upset as he paced the motel room.