Dean remembers what it felt like to be Lucas.
"Kids are strong," he tells Andrea. "You'd be surprised what they can deal with."
He remembers what it was like to be a kid, thrust into a world that was too hard for his dad to deal with so how could Dean, who was only four, after all, be expected to deal with it? At four, Dean didn't even have a real concept of death, and then, suddenly, there was his mom, on the ceiling, screaming, bleeding, dying.
Dean asked for days when she was coming back before John broke and yelled.
"Never, Dean!" John snapped. "Mom can't come back. Nothing is going to change that, so stop asking!"
Dean cried after that, and didn't speak unless spoken to for months. Didn't speak at all, for months, because John didn't have much to say either. But he made it through just fine, didn't he?
Granted, Dean's pretty sure he wasn't being used as some sort of psychic conduit for whatever killed his mom the way Lucas is for whatever killed his dad and his haunting the lake, but it's the same basic principle.
When Dean was four and John yelled at him and he couldn't get the smell of smoke out of his pajamas no matter what John tried, Dean felt like he was suffocating, like he was still trapped in Sam's nursery, looking for John or Mary or Sam, or anyone who would help him, and instead catching the tiniest, briefest glance of the most horrific thing he would ever see as long as he would live and being handed a baby to take care of forever. He was too little, too upset, too scared to articulate any of what he was feeling. Dean didn't even totally understand what had happened, only that his dad was upset and his mom wasn't there to kiss him goodnight anymore, and Sam had a cough that made the crease in John's forehead as deep as the whole Dean dug in the backyard the day before the fire.
Things are different for Lucas, sure, but that doesn't stop Dean from feeling just like he did when he was four when he Lucas clings onto his arm in the police station after Bill Carlton died. Like he's suffocating (or in Lucas's case –drowning) and there's not enough words to ever explain what he's feeling. Probably, Lucas doesn't know what it is that's got him feeling like the world is ending, but the entire way out of town, Dean can't shake that look. Lucas's whimpers. The look on Andrea's tired face as she dragged him off of Dean, apologizing, at the end of her wit.
And that's why Dean turns the car around, even though Sam thinks that the case is over, even though Sam is still hell-bent on finding Dad and damning the rest of the world if he has to. And it's not that Dean would turn the car around for every scared kid, then they would have never left California. Hell, they would have never left Kansas. But…well, Sam said that sometimes after traumatic instances kids (people, but especially kids) sometimes get in touch with the other realm, the veil, or whatever they like to call it.
And the drawing the houses of exactly where Sam and Dean needed to be.
And, if Dean is being honest, how much Lucas reminds him of himself in the weeks following his mother's death. And how much Dean wished someone had told him that it was okay to be scared. How being scared was the first step in being brave. How much he wished someone could have listened to him, tried to understand him, when everything that was happening to him was so far outside his comprehension.
Sam wouldn't understand. Sam is taking his grief another way. Burning down the entirety of the continental United States would be worth it to Sam if they got Dad at the end. But Dean is trying to help people, dammit, and he's going to start by helping Lucas.
Even if all he can do is check on him before they leave this Wisconsin town behind them forever.