It's where Blair's language and Dan's language meet, because they're both afraid of the same thing, which is that somebody will notice how easily they can be destroyed. He doesn't even stutter. "You know, the first time that I told Serena I loved her, it was terrifying. I've never felt so exposed. But the feeling that I got when she said it back to me... was probably the single greatest moment of my life." Blair points out that they broke up, but even Dan knows that's beside the point, and he'd do it the same way again. "If I say it, he wins..." Dan shakes his head, knowing she's on autopilot, scared out of her wits in a shitty Brooklyn café. "...And if he wins, then I'll just be another girl to him." The last thing, the very last thing, Blair could ever be. "You don't know that that's true. You have to decide what's most important to you: keeping your pride and getting nothing, or taking a risk and... Maybe having everything." They lock eyes; it's the first time they've ever seen each other.
It's awesome because here's the math. There are things that Serena has always done for Blair (make her feel brave; make her feel loved), and things she's always done for Dan (make him feel seen; make him feel known). And there are things Dan's always done for Serena (make her feel strong enough; make her feel real), and things Blair's always done for Serena (give her a place to land; fight for her to the death). And as long as Dan and Nate are slipping into and out of each other's lives and stories, trading names and faces whenever it suits them, bouncing Chuck between them like a grenade about to go off, let's impute Dan into the Blair/Serena thing and see what happens, because what happens is that -- for this moment -- the equation works out beautifully. The thing Serena loves about Dan is the thing Blair desperately needs without knowing it; the thing Blair loves about Serena is the thing that he's always been able to give.
Blair gives Dan the acceptance he's always craved from her and from the part of her he sees in Serena (in his backwards sour-grapesy teen-boy way, but still needs and craves) and meanwhile all Dan's saying is the magic words: "You're doing so good! You got it! Look how good you are!" Would the show really fall apart if it stayed this way for awhile? These two freaks need all the help they can get, and it makes me happy when anybody can see through or around the very substantial layers of bullshit they generate around themselves every day. Blair deserves to feel brave, and loved, and Dan deserves to feel like he's helping, like his help is actually requested and required. Like a man.