The reason he got tired of Blair for a second at the van der Bilts was because she was becoming too much like him, which adds up to way too close to the abyss, even for him: "To prove that nothing matters" is exactly what he's afraid of: that it doesn't. And if that's true then he'll have legitimate grounds to actually, literally go crazy in that Less Than Zero way, and he'll end up Carter Baizen, or Pete Fairman. He needs somebody to stand at the border and tell him when he's getting too close to it, and that was Blair. Except for when she jumped in with him, which has scared him to death every time she's done it, because -- compare, if you like, to Georgina's complex dance with Serena last year -- he knows she needs him for the same reason: to show her where the edges are.
Vanessa's carrying a gift, and Chuck looks amazing in his suit; they spot Blair fussing with Nate's jacket playfully, like old friends, and when she spots them she pats Nate's shirt pocket, alerting him smoothly to the new thing. The two mismatched couples stare at each other for a second, and Vanessa's like, "Fuck! What do we do?" She grabs Chuck and kisses him; it starts out awkward and fake, but turns into something else, as Nate and Blair stare, looking like they've just been hit in the head with identical dumptrucks. "Spotted: Two people who should know better... And two people who don't know what hit them." But which are whom?
Now it's a rager indeed; some cute guy wanders past with a Lily fishbowl full of weird bead strings as some guy's harassing Serena about the lack of keg. Poppy comes up, once again disappointed in Serena's ability to play with the big dogs and asking who all these "rowdy children" are. Even if Jenny's just sixteen, these people are ridiculous. Serena apologizes a million times, but Poppy and her entourage are done. She invites Serena along, but S knows she needs to deal with the trouble first. S offers to catch up later, and Poppy tells her to really try, for her own good. Seriously, S needs a drink far away from here. Everybody could use one, I think.
Serena continues to feel idiotic as Poppy leads her beautiful crew out the door, and then snags Jenny, asking her to help herd the crashers out. "Um, they're not crashing, Serena. I invited them?" Serena stares at her in total incomprehension, and Jenny says they're standing in the midst of her guest list. Serena points out this no matter what, this is not what Jenny could ever have wanted, and Jenny -- loving every second of it, in a really unattractive but totally identifiable way -- goes, "Like you care what I want!" Serena flips into Lily mode again, saying Jenny's name and swearing she "genuinely thought" that Jenny's socks would be rocked when she got off the elevator, and if they weren't, "Then I'm sorry. But you should have just said something." Which I swear is a conversation she's had verbatim with Lily, or CeCe, or both.