They're family and they love each other. That matters more than their differences. They grew up very close and Dean wouldn't change that even if he could. He had a chance (sort of) to change it with Carmen and he didn't.

There is nothing that Dean wouldn't do to give Mary (and the Winchester men) those years back. (Maybe that's Dean's problem or maybe he was the influence. John had actually worried a little about it, but he was smart enough not to say anything). It does ease his unfounded guilt when she says it's okay that he doesn't remember. He hesitates a moment and takes a sip of his coffee. "I--I remember you dancing around the kitchen with me." His brow furrows. "The Beatles...maybe?" He looks up at her, eyebrow raised in askance.