Everybody's having fun at the party, until Chuck surprises B again with puppy love in his eyes. She's like, "Stalk me much?" and he asks why she's still all alone at her own party. Which could be the title of her autobiography, really. She explains that Nate still hasn't shown up, but he always calls at midnight, so she's waiting. Chuck is like, "I bet he doesn't, and also, isn't it totally weird how he only reconciled with you the day after his dad totally went to jail and imperiled his family twice as bad as before?" So yeah, he's acting out of self-interest, but that self-interest is...I'm not making excuses for him, but it's clear that he loves two people in the world: Nate, and now suddenly Blair. This is the only way for him to have them both, of course, but it's also the only way both of them win as well. Look: people only ever act in their own self-interest. The brain is not wired to do anything else. There's nothing wrong with that, it's just a fact; addicts do it in the short term, just like martyrs, and regular nice people do it in the long term. But if your self-interest is sufficiently big enough to include the welfare of other people, you can do both without selling yourself out.
Look at Nate: he's torn between acting as an Archibald, which includes love of his family and love of Blair, and acting in only his own interests, which involves...he doesn't know yet, because he's a kid, but he knows he's getting dicked around, so it involves not getting dicked around and that's as far as he's thought about it. So by running off with Jenny, he's playing a short-term game; he literally has thought as far as not going to the party, so he doesn't. Meanwhile, Chuck is hurting the only two people he loves in this entire world, in order to get some shit accomplished, for all three of them. In this particular instance, then, Chuck is better than Nate. (Of course by that logic, that means Vanessa is better than Dan "What A Coincidence" Humphrey this week, with all her advice and meddling and inviting herself places, but..."better than Dan" is kind of neck-and-neck anyway, so I concede.)
"So you're saying that Nate is only pretending to like me, and that he's actually using me to get to my mother? He wouldn't do that." Chuck's like, "Except that the reason that you and I -- and our recapper -- are completely in love with him, the secret awesomeness of Nate, is that he would die for his family. Trying to do the right thing, something nobody in the UES is equipped to do, is going to tear him violently limb from limb, and you are the cross he's nailed to." B agrees, but can't say that out loud without losing the fight, so she's like, "In other news, he is at least going to call me at midnight, because he loves me." Chuck offers to make a friendly wager on that, and she -- again -- can't lose the face necessary to back down, so the wager is this: Nate doesn't call, they fuck. Nate calls, Chuck crosses her off his rape list. She musters up her courage and faith, in Nate and God both, and promises Nate's going to lose: "He's never missed my birthday." Of course, without Blair being in a pissing match with God, we would have no show, so Gossip Girl's like, "Careful, B. There's no safe wager when you bet on a Bass. You just might lose your shirt...and your pants!"