When Sam was twelve, Mrs. Sage informed him and his brother that they shouldn't say "ain't" because it made them sound uneducated.
Dean glanced down at his oversized leather jacket and beat-up boots, looked her dead in the eyes, told her, "Ain't like you're any better than us", and walked away.
If anything, Dean said the word even more after that conversation.
Sam stopped saying it altogether.
(There was a lesson in there, somewhere.)
The towns his dad dragged him and Dean through tended to be small, rural, and in states where nobody really took notice of how you spoke. Mrs. Sage of San Jose, California was the exception.
Sam learned to pay attention to how he spoke after that. He hadn't even realized he had an accent until he started to focus on eliminating it.
Dean gave him weird looks when Sam stopped mid-sentence to correct himself. When Sam progressed to the point where it was instinctive to correct Dean's grammar as well, the weird looks turned to hurt ones, and then to angry ones. As a teenager, Sam convinced himself he was doing Dean a favor, that he was helping his brother be better.
Once he had some distance, Sam would feel guilty about that. Still, at Stanford, he couldn't help but be thankful for Mrs. Sage's words, however hurtful they had been; if he hadn't known to speak properly, he would have been even more of an outsider. For the most part, his classmates came from the coasts and from families with money. Sam, with his secondhand clothes and battered textbooks and single duffle bag, was noticeable enough without sounding like a hick.
Despite his best efforts, Sam's accent seeped through when he got tired. Jess found it cute. He found it embarrassing. He'd run two thousand miles and twenty years and still couldn't outrun his own voice.
Dean left a total of three voicemails during Sam's time at Stanford. Sam listened to each one on repeat for days on end, but he never called back.
He could hear that same honeyed drawl in his brother's voice, and in it he could hear eighteen years spent blowing through small, dusty towns.
Sam could tell how easy it would be for him to fall into that life again. He couldn't risk it.
He hadn't said "ain't" in nine years by the time Dean stopped calling.
Two years after he left Stanford with Dean, Sam got thrown through a window. Dean spent half an hour picking glass out of Sam's back.
The next morning, Dean went to pick up coffee. He paused in the doorway. "You gonna be okay until I get back, Sammy?"
Sam mumbled, "Ain't like I'm gonna die in my sleep without you, Dean."
His face was buried in his pillow, so Sam couldn't see Dean's face, but there was a long pause before his brother responded.
"Go back to sleep, Sammy."
Sam thought about objecting to the nickname, but he was too tired to muster the energy.
When he woke up again, there was coffee on the nightstand, and Dean was singing a bad country song at the top of his lungs in the shower.
Sam smiled.
Dean's voice sounded more right to his ears than anyone else's ever had.