I'm doing a rewatch & we've just hit Season 7. Meanwhile, one of my favorite fanfic authors is on S7 of her ficlet series! And S7 is really not my favorite, but I gotta talk about Dean here... and what the heck the writers were doing.
[See also: Re: Dean Killing Amy (and lying about it)]
The first time I watched it, S7 felt kind of incoherent. Characters kept harping on about Dean's mental health/state of mind, but (unlike when he came back from Hell) I wasn't really sure why he was falling apart — and I didn't really "see" it, either. SPN's episodic structure can make it hard to keep track of the emotional through-lines of its protagonists, and the writers didn't always dedicate enough time to reinforcing our intended takeaways. It wasn't until I rewatched S7 that I felt like I was beginning to put together all the pieces. When I did, here's what I saw.
S6: A New Dean
First, we have to go back to S6:E1, where we meet a new Dean. He's a year into healing from the loss of his brother and, while he says he never gave up on saving Sam, it's clear from the montage (which doesn't show a drop of his search for answers, despite echoes of Sam continuing to weave themselves through everything he does) that, months down the line, the edge has worn off his grief. He might still be looking, but he's not utterly consumed by it. Not anymore.
He's got good days & bad days, and getting up in the morning & facing the world is still work, but for the most part, Dean is settled into a fair approximation of a decent suburban family life. He's not waking up & immediately reaching for a beer. He's not carrying around a flask. His nightstand isn't littered with empties, and he's definitely not gulping straight from a bottle of Jack. (Which is to say: We've seen what it looks like Dean's mental state gets really bad... and this isn't that.)
Maybe he still sometimes has a few too many before bed — we can probably extrapolate that from Lisa's truth-cursed questioning of him drinking "half a fifth a night" to keep dark thoughts at bay (of course, it's possible that that was a more accurate description of when he first came to live with her: Veritas made people speak the truth, but not the whole truth, after all) — but for the most part, he seems to keep it to one or two after work, a beer while he's grilling, a nightcap before turning in. He doesn't get sloppy around Ben. He doesn't drink on the job. He's... y'know, managing.
He's also not out there flirting & womanizing like the old Dean did. He's got steady work. He's faithful to his woman. He's basically turned into a responsible adult.
Dean reacting with a mixture of amusement & consternation to a waitress's flirtation.
Some part of what we see may be a role he's playing: Dean is naturally adaptive to his environment (as we saw in "Folsom Prison Blues," "Hollywood Babylon," and even "99 Problems"), and the suburban family life could be just one more scene. But a good chunk of it seems to be finally just having a little peace & quiet — and a little time & space — to grow up, to heal, to settle down.
Things Get Hard
Dean continues to stay on the straight-and-narrow even as he gets increasingly suspicious of & jumpy around his newly-returned brother. It's not until S6:E6 "You Can't Handle the Truth" (following Sam throwing him to the vamps & Dean's subsequent disastrous missteps with Lisa/Ben) that certain bad habits start creeping back in.
We see Dean call Bobby about Sam, saying, "I don't know how much longer I can do this" and "my skin crawls just being in the same room with him," and Bobby reluctantly telling him that the search for a supernatural explanation for Sam's behavior is turning up nothing. That this might not be anything that can be cured: it's possible this is just who Sam is now. And Dean is suddenly staring down the barrel of a future that might not contain a family as he's known it in either Sam or Lisa & Ben.
This is the first time we see Dean drinking again to cope with stress: he grabs a beer while venting to/pleading with Bobby, hangs up, takes a long swig, and steels himself to call Lisa (presumably to try to apologize)... and then loses his nerve & hangs up again.
Afterwards, Sam connects with him about the case & Dean ends up pursuing a red herring for a bit. When Cas appears but can't help him with Sam (but confirms he's not Lucifer, forcing Dean closer & closer to the unpleasant conclusion that this is just Sam now), Dean's drinking escalates to whiskey.
After Cas disappears, Dean hits up a bar, but we can tell he's not accustomed to just chugging shot after shot anymore: when the bartender asks if he'd like another, his knee-jerk response is to turn her down ("No thanks, I'm working") before Sam's call — & dread at the prospect of spending more time with him — sends him back for one last drink...
