The Way Things Are
Dean hated it when Sam tried to look out for him.
He wasn't too prideful to accept help from his little brother. It wasn't that he didn't know if Sam could handle it; he'd been with his brother for almost his whole life, he knew Sammy wasn't just capable, he was good. And it wasn't even that Dean felt his brother was worth more than him, that Sam shouldn't risk his own safety to protect Dean.
It was the way Sam said "you would do the same for me" that cut him.
When Sam tried to rationalize it like that, tried to shrug off all his stupid nobility like it was the least he could do, it almost made Dean want to scream. Because Sam didn't- couldn't- feel the need to protect his brother like Dean did. He didn't doubt Sam's love or selflessness, but it was the truth. All of Sam's struggles and turmoil and pain that he went through for Dean's sake, that was all of his own choosing. And for Dean... it wasn't.
It wasn't out of obligation or choice or love that Dean felt this way, it was simply the way things were. Like something would be fundamentally wrong with the universe if things were different; grass is green, water is wet, and Dean Winchester protects his little brother. It was as simple as that.
There was never a question of what Dean would or would not do for his brother. He would do anything, anything for Sam, without the slightest hint of hesitation or forethought. Dean knew he didn't value himself as much as Sam wanted him to, as much as Dean himself knew he should, but that was never a factor. Even if he had thought he was the best person in the universe, he'd still give up everything at the drop of a hat if it would benefit his little brother.
Sammy came before everything and everyone, and it was unthinkable for it to be any other way. There wasn't any choice involved here, and it hurt Dean to know that Sam could never understand that. He could try to, but he knew what Sam would say. He'd either downplay it as Dean trying to be humble and not accept what a great big brother he was (yes, great, tell me Dean, how many times have you broken his trust and his heart?) or, worse, assume it to mean that Dean's actions were out of compulsion or duty, that he didn't really want what was best for Sam and was just following orders once again. And it wasn't like that, it was never and would never be like that. Dean loved Sam more than anything, more than himself, more than all of creation combined.
That was why he had no choice. Even if he would, he couldn't. And even if he could, he wouldn't. Dean would tear apart the very essence of the universe before he put anything in front of his brother.