I will follow you into the dark.
It's the middle of the night and Dean is running. He's not running from anything, not really, and although he has a destination, he knows he will never be able to stop.
This is something he does now, when he can't sleep; he's done it innumerable times, although if he really wanted to know, to count them all, he would only have to pull out a calendar and count back to That Day because fuck, he hasn't been able to sleep a night since without running himself into the ground, then going home and drinking himself into a stupor. He knows Lisa can see what he's doing to himself; knows that it hurts her to see him like this and not be able to help him. He wishes with all his heart that he could be somewhere, anywhere, but here.
But Dean made a Promise, and that's promise with a capital 'P' as he used to tell Sammy when they were little and the best way to make something really feel important was to spell it with a capital letter. And it's the second-last thing he promised his brother, his Sammy: the last being that everything was going to be alright, because he was there. He can't be sure that Sam heard it, but he knows what he promised, and he knows that he broke that promise.
This is why he keeps torturing himself.
If it had been his own choice to come to Lisa, he would never have stayed. Hell, he probably never would have even showed up in the first place. But he promised Sammy (with a capital 'P') and the least he can do after breaking his last promise to his baby brother is keep his second-last promise.
And so he stays with Lisa. He stays here and he pretends to live a normal life, pretends that most nights he wouldn't rather keep running into the night instead of turning back.
Pretends his heart doesn't break a little more each day.
If Heaven and Hell decide
That they both are satisfied
Illuminate the No's on their vacancy signs
If there's no one beside you
When your soul embarks
Then I'll follow you into the dark
Then I'll follow you into the dark