I chose to respond to link
文章 via another 文章 because the 评论 would be too long. I suggest 你 read that one before 阅读 this one unless 你 intentionally don't want it to make sense.
你 do realize that Blair is the one who always called Dan for help right? He didn't pull her away from anyone. She distanced herself from her other 老友记 because the only person whom she felt could help her in any way was Dan. She could be herself with him. "You were the only one who could save me from my own worst instincts," (5x03). I'm pretty sure this was made clear throughout all of season 5. The only time she looked for help from someone else was in 5x04 with Serena when she believed her friendship with Dan had ended after the whole book thing.
Dan didn't make any moves on her. He was genuinely being her friend because he cared about her. He could never abandon Blair and leave her to her luck. That's not what a true friend would do.
What do 你 mean Chuck was Dan's friend? Dan was HIS friend. Dan helped Chuck through his whole 'I can't feel anything' phase. He even tried to get Blair to talk to Chuck. The only reason Chuck was so worry free when he found out about Dan being in 爱情 with Blair in 5x08 was because he believed she loved Louis and Dan would never stand a chance anyway. In 5x16 Chuck tried so hard to ruin him and I quote Dan, "You wouldn't be trying this hard if I didn't have a real shot with her. And up until this moment I wasn't sure if I did. So thank you."
由 the way, Chuck paying the dowry was basically him cleaning up his own mess. It was his fault that it needed to be paid for in the first place after sending the Dair 吻乐队（Kiss） to Gossip Girl. Also, he was filthy rich. Dan would've payed Blair's dowry if he had as much money as Chuck.
Blair had every reason to think Chuck had ulterior motives for paying her dowry especially after everything she's been through with him. He has always done these GRAND gestures after she chooses someone else over him.
I do believe Chuck was being genuine all those times though. In 5x19 he was just looking for a friend in Blair, but everything she 说 about them not being good at being 老友记 is true. They make great scheming partners and sex buddies, but friends? Not really. Overall, they don't have that much in common.