It was well-understood that Dean Winchester was lactose intolerant.
Not lactose intolerant like he couldn’t consume milk products or he’d need to camp out at the gas station bathroom for four hours, but that he did not tolerate milk. He despised the white liquid, and spent most of his childhood pouring his glass into Sammy’s.
(Please, like Dean would actually allow his body to have an allergic reaction impeding him from his Chocolate Cream Pie.)
For the earlier part of their childhood/hunter-life, milk was hard to come by on the road. Milk did not keep well in the Impala, and the occasional chance at the closest Gas ‘N’ Go was usually replaced by root beer or just lots of bottles of water next to Sammy’s car seat in the back.
(For a while, when Sammy was still getting weaned off mama milk, they had baby formula in the trunk next to the shotgun and axe, but little Sammy got sick and tired and wanted his mommy and threw up all over the back seat. John put his foot down, and that was the end of that.)
Once they started settling down a bit more, staying long-term in cheap motels and enrolling in school for weeks or months at a time, milk would grace the small fridge beside the bed. Most meals were at the local diner but John, although not the greatest hunter yet, was still an alright father, and remembered to order chock-full-of-calcium milk for his two sons. Dean remembered most of those nights switching his glass with Sammy’s once his brother guzzled his down.
One time, Dean was about to run out to the store for some snacks, when Sam asked him to bring back a half-gallon of milk. Dean made a face. “You get it yourself.”
Sam didn’t look up from his writing but he replied, “I can’t. I’m doing my homework.”
“Oh yeah, when’s it due?”
It was Monday. That little nerd. “Fine, but you’re drinking the whole thing. Dad’s about to come back any day from hunting and then we’re leaving. Don’t make me waste my money.”
Sammy rolled his eyes. “Right, your hard-earned money that you took from the man at the bar.”
“Won, Sammy. Won.”
Sam didn’t roll his eyes again, but Dean caught the little smile he tried to hide. “Why do you hate milk so much anyway?”
“I don’t hate it, I’m just leaving more for you. You need it to grow big and strong.”
Sam pouted. “You’re still growing too.”
Dean grinned and ruffled his little brother’s hair before grabbing the keys off the table. “I’ll always be bigger and stronger than you Sammy.”
Nowadays they sit together at a diner, like the good old times when dad had gone hunting and they were sharing a dinner of spaghetti and omelettes, except Sam is on his laptop and Dean is downing a burger.
Sam is slouching in his seat, but he is still taller than Dean.