Dean’s life had really crumbled to pieces after the final showdown of Michael and Lucifer. He lost his brother. The only other person to truly understand the parent that John Winchester was. His brother. Down in the cage, Sam was somewhere Dean simply couldn’t be.
So he started a life with you. You helped him pick up those pieces. A plain old apple pie life and he loved it. He loved the ragtag family he formed with you and the plain old suburban house he now dwells in. He has stability, something he never had before.
He has one goal; Don’t screw this up.
The goal remains, but now that Sam is back—albeit different and distant—Dean feels an irresistible pull to start hunting again. Its the way of life for Dean, it’s in his DNA.
The way of life for anyone, is all good things must come to an end.
Dean knows he’s become paranoid, unnecessarily stern, he knows he’s become quick to snap at you, because he is fighting tooth and nail to make sure this good thing doesn’t end.
To do that—he has to do something infinitely harder than juggling hunting and regular suburban life. He has to talk to you. He pulls you aside to the kitchen, he’s leaned against the counter top as you stand beside him.
“I know I’ve been an asshole lately…I’ve been…Hell, I’ve been a coward.” He whispers his expression is withdrawn, as if he’s reminiscing with bad memories.
“…You tell yourself, you’re not gonna be something.” He shakes his head almost shamefully as his eyes eventually meet yours, “But my dad…was exactly like this. All the time.”
His shoulders drop in defeat, and his head hangs low, he’s disappointed in himself. “Used to scare the hell outta me.” He whispers.
He didn’t want that for you. You didn’t deserve John Winchester 2.0, you deserved better.