What I feel from the relationship is this:
They are basically dating an opposite gender version of themselves.
Dan & Nessa are te same type of people, Chuck and Blair are the same type of people, Serena and Nate are the same type of people.
Look at there thought, view, what they do. They are very similiar.
What I see from that is: No one wants to date a female/male verison of themselves, at least I wouldn't. Yes we have to have the same beliefs on things or else we would clash, but you wouldn't want someone who is like you and agrees upon everything. That's too boring for me. I want an opposite, but someone I can torelate. Which is why I like DB. They're completely diff, but they're always there for each other when they are down.
But like I said i just started watching so I may be wrong, I apologize. D/J siblings, wow they don't act like it. I agree, Dan needs to step it up on that.