I wanted to go into a little more detail about what I meant by agreeing with Dean's arguments but not his conclusions. Let's start with Dean's explanation of why he doesn't believe in Sam: "I don’t know whether it’s gonna be demon blood or some other demon chick or what, but…I do know they're gonna find a way to turn you.[...]You’re angry, you’re self-righteous. Lucifer's gonna wear you to the prom, man. It's just a matter of time."
First, I want to talk about the idea that Sam would be turned by demon blood. Sam was addicted to demon blood, no doubt. He craved it. But once he stopping drinking it at the end of S4, he had no slip-ups, except in the famine episode. Note that after the famine episode, even though he’s had it again, he has 0 problems. While Dean doesn't know about this, Sam even had demon blood shoved in his mouth in Free to Be You and Me, and spit it out without issues.
After PoNR, Sam will take it in order to be possessed by Lucifer as part of the plan to stop him. And as far as we know, he never takes it again. (At least not through the end of S10, which is as far into SPN as I have seen.)
Admittedly, this is probably a symptom of the unrealistic portrayal of addiction in television shows. The battle for sobriety in these types of shows is usually far easier and simpler than getting off drugs in real life. But within the context of the show, Dean in PoNR has been able to observe Sam for most of S5. He was certainly around for the famine episode. He has the information to know that addiction was never Sam's primary reason for drinking the blood, and he has the information to know that at this point an addiction to demon blood is not Sam's weakness.
Sam's erratic behavior in the Rapture proves that he was truly addicated, as does his disintegration in that episode and the next when he was denied the blood. The addiction certainly was in Ruby's interests, as it increased his dependence on her.
But Sam did not stop drinking the demon blood because of the consequences of the addiction, and his addiction wasn't why he started drinking it again in OtHoaP.
Sam drank demon blood to defeat Lilith. At least, that’s what he said. ("I'm not drinking the demon blood for kicks. I'm getting strong enough to kill Lilith." That is line that will never not crack me up. S4 Sam in a sentence: He doesn't want candy, and he's not drinking the DEMON BLOOD for KICKS.)
That's true, as far as it goes. But it’s more complicated than that. Sam drank demon blood because it made him feel powerful. All his life, he had been manipulated and controlled by stronger entities. He’d been stalked by a demon - and, as it turns out, indirectly by the devil - since before he was born. His life had been shaped by the beings who are trying to turn him into exactly the thing he wanted to be least: an evil monster.
Sam rebelled every step of the way. He fought for his independence. He fought for revenge, for love, and simply for himself. And every step of the way, the choices he made to maintain control over himself and his life backfired massively. He spared Jake to keep from turning evil, and he ended up with his brother promised to Hell. He did a 180° from his previous noble mercy, killed Jake, and spent S3 trying to be more badass...which got him exactly nowhere.
Sam wants power. He hates feeling powerless. Sam wants control. He wants to be his own man. Sam wants something he can do. As I said in another post, he's action boy.
So, yeah, the demon blood was appealing.
But the appeal of demon blood for Sam in S4 is the exact reason why it became completely unappealing in S5. At the end of S4, Sam saw that demon blood had made it easier for Ruby to manipulate him. In drinking the demon blood, Sam once again played right into the hands of the people trying to control him. Dean wasn’t entirely wrong when he told Sam, in WtLB, that drinking demon blood was weak, not strong. After breaking the final seal, Sam no longer saw demon blood as a way to escape the trap of his life, as a way to get revenge, as a way to protect himself and his loved ones. Instead, it was yet another way for the bad guys to get to him. And as with his 180° when he killed Jake, Sam did a 180° about the demon blood. The primary issue for Sam wasn't the addiction. It was the power, and control over his own life. And demon blood no longer seemed like a way to get that.
As for Dean's comment about the Devil getting to Sam with a demon chick, that is just eyeroll-worthy. It’s as if Sam has some kind of special weakness for demon chicks. It sounds a bit like Dean’s shitty line in Sex and Violence about Sam banging monsters (“No, It's just...first it's Madison, and then Ruby, and now Cara. It's like... what is with you and banging monsters?”). You might as well say that Dean has a thing for demon chicks because of the way he clicked with Casey in Sin City.
Sam’s relationship with Ruby was not about having a thing for demons in female bodies. It was because she offered him what wanted: knowledge, control, Dean not in Hell, Lilith dead, the apocalypse stopped, instructions on how to use his powers, etc. She made good on a lot of the promises, and was at other times very upfront about what she couldn’t do (saying in I Know What You Did Last Summer that she didn’t know how to rescue Dean, for example). The fact that she was upfront about many negative things - refusing to tell Sam her motives in early S3, admitting to being manipulative in No Rest for the Wicked - probably made her seem more trustworthy to Sam. Ruby didn’t win Sam over because she was a demon, or even because she was female. Sam’s eventual trust in her was a long story with a lot of ups and downs, and a lot of different factors contributed to his choices. It was, in short, a brilliant mind game that took lots and lots of patience. Ruby was nothing if not patient. It wasn't about some kind of fetish for demonic women.
Dean's arguments in PoNR for what would cause Sam to give into Lucifer were about Dean's personal issues with Sam, not Sam's actual weaknesses. But his conclusion, that Sam would say "yes" to Lucifer, was absolutely correct. Sam not only does say "yes" to the Devil, but he says "yes" in Detroit, the very city where Lucifer predicted it would happen.
It's not demon blood the motivates Sam. Or "demon chicks". It’s all Sam’s classic weaknesses. Self-loathing, confidence, anger, determination, a willingness to go all-out to achieve his goals, and the simple fact that he’s action boy. He needs to do something.
Sam comes up with a crazy plan wherein he will say “yes” to Lucifer and then he will overcome Lucifer and jump into a cage in Hell with the Devil and be tortured for all eternity.
As others have pointed out, Sam’s plan to defeat Lucifer displays way more arrogance than he does at any point in S4.
And also self-loathing.