You get a two-fer.
The taking back of Dean
It puzzles me that the Dean vs. Dean scene is being taken as breaking Dean down, as being about Dean hating himself, because the scene is the opposite, it's him fighting for his sense of self, and winning. Dean seizing Dean back from the darkest parts of his brain. You might want to just go read
janissa11s
post on 3x10, I think her take on what's going on with Dean makes a lot of sense, and she puts it much better than I can.
On the John thing, here's how I've got it sorted out in my head. John created this in Dean. He made a mistake, he put too many burdens on Dean too young, and then he went and did it again with the secret about Sam. Dean's resentment is justified and it's tragic it took Dean so long to acknowledge it. You had him being the good soldier through childhood, into adolescence and adulthood, and when John took the burden off his shoulders (by acknowledging his mistakes and praising Dean) and then put it back on again (telling Dean to look after Sam, yet again, and that he had to save Sam or kill him) in IMTOD, it put a fresh set of weights on Dean's back. With John's dying to save Dean thrown into the blender.
The Dean vs. Dean wasn't Dean realizing John didn't love him, or loved Sam more. This was Dean acknowledging his resentment of John because John made him feel this way, Dean's acknowledgment that he's worth more than that. The other!Dean was speaking all of actual!Dean's deepest, darkest fears about himself and about what John thought of him. Fears, not what Dean really believes about how his father viewed him. But the fears didn't come out of nowhere. Dean saying John was an "obsessed bastard" is a Dean-pov truth but also some actual truth. No matter how you slice it, John was obsessed, and at times he acted like a bastard. (I love the guy, but I called him a bastard in an ep reaction last season.) It's not the full picture of John but it's the part of John that's hurting Dean.
And no, I'm not John-bashing, and I'm not John-apologizing. Does it always have to come down to John-bashing or John-apologizing? I adore his character, I think he's fascinating and I think canon gave us as much evidence of his strengths of a father as his weaknesses. But he wasn't aces as a father in many ways. None of John's mistakes come out to maliciousness or because he didn't love Dean or because he didn't want to do 110% right by his children. He loved Dean, and Sam, too much. He wanted to avenge Mary, but his greatest fear was he'd lose her sons. Which isn't to excuse John, but for Dean, the reasons aren't a comfort. What was done, was done.
Dean saying to demon!Dean how it's John's fault Mary's dead, because he couldn't save her, is particularly telling.
batyatoon pointed out that Mary dying is Dean's single greatest proof that his father isn't the all-perfect hero. And back in early season two, we saw how angry Dean was at John for trading John's soul to save him, for leaving, for being mortal. Dean says to Sam, he should've gone out fighting, not bargaining but I think even if John had gone down fighting, Dean still would've reeled the same. Then there's Dean's anger at John making a deal, and Dean saying he shouldn't still be there -- because Dean thought he wasn't worth the bargain. I don't mean he literally thought that, not consciously, but the dark corners of his brain thought so. (BTW, I also made
batyatoon explain the demon!Dean snapping his fingers at the end, because I didn't know what the heck to make of it, and she said it's to remind us that the possibility of Dean going demonic -- going to hell -- is still there. I also thought maybe it was a reminder that just because Dean's confronted his inner emotional demons, and won that round, they won't go away just like that. If you take demon!Dean as both a representation of what Dean could literally become if he goes to hell, as well as representing his, well, inner demons.)
Dean's journey to the scene with his demon self didn't start in season three, when he made the deal to go to hell, it was already going on in season two. It started long before that. It started when John put baby Sam in his arms when Dean was four.
The payoff here was Dean saying "I don't deserve to go to hell." Not "I don't want to go to hell" or "going to hell will suck beyond the telling." But "I don't deserve that." He tells Sam in the coda scene "I don't want to die," which is the other thing I really wanted to hear him say. But the "deserve" thing had to come first and was core here. Dean could want to not die but still think he deserves his fate.
those three little words
Lisa saying "I love you" to Dean struck me strongly. I think this is the first time in the entire run of the series that anyone has said "I love you" to Dean (or is my memory off on that?) I'm not sure anyone has said it to Sam, either (did Jess?), and I don't think Sam and Dean have said it to each other in those exact words. They don't need to say it, and they do say it in every way short of actually saying the words.
So then I got to thinking about when the phrase "I love you" has been said in the show and can only think of two. One is in Crossroad Blues, the wife says it to the husband who made the deal, and her face distorts into a scary death harbinger. The other time is the girl in CSPWDT who says it into her cell to her cheating boyfriend, right before she dies. I'm sure I'm forgetting others.
But it would seem, on this show, that the words "I love you" are rare and associated with loss. Because in the Supernatural 'verse, if you love someone, they die. The look on Dean's face when Lisa says it, he just looked, startled, almost stricken -- and I wonder if anyone's said it to him since he was four. (And please, yes, tell me if anyone has said it to Sam or Dean on screen before now. It'd make me feel better.)
This is only speculation, but it's good odds John didn't say it to either of his kids because, to John, if you love someone, they will die. It's not safe to say it. And how much of that did he transmit to his children?