I used to be a hardcore Chair shipper, but after season 3, I just couldn't do it anymore. There are so many things that went wrong with Chair (and Chuck).
He basically chose Bass Industries over Blair, causing her to almost sleeping with Chuck's uncle; he went ahead and slept with Jenny Humphrey (Blair's enemy) moments after waiting for Blair because he apparently loved her so much (wow u didn't show up so imma sleep with ur enemy oops!!!!!!); and he almost hit her following that event.
And you know what's sad? Blair always went back to him. Always. I am sorry but Blair always tried to come off as the "Queen Bee" and a girl who's "independent." But every time she went back to Chuck, I used to shake my head. Like what utter bullcrap. I was almost expecting her to go back to Louis after everything he did (because seriously I wouldn't be surprised, she's done it so many times with Chuck), but alas! She used her brain this time!
Dan may not be the most flawless character in GG (no where close since he falls for girls way too easily), but he's definitely better for Blair at the moment. He'll always be there for her - as a friend or as a boyfriend. He doesn't need to plot to make Blair his as if she's some kind of inanimate object (*ahem* Chuck who basically did so, leading to much trouble for the Waldorf family). And he actually wants to see Blair happy, that's why he sent in the video during Blair's wedding, thinking Blair's happiness is with Chuck.
I think Blair is a very fickle-minded character. She went from "I'll always love you, Chuck" to "You don't have my heart anymore" in a matter of only a few episodes, I agree. However, I still think Dan is the right guy for her. At least at the moment.
I know Chuck has apologized, but apology doesn't heal all wounds. If someone went ahead and killed a guy, and then said "sorry," what does it really do? Nothing. Episodes after Chuck said sorry, he went back to his old scheming ways - sleeping with Dan's boss, wanting to ruin Dan's career, sending in that text to Gossip Girl which might leave the Waldorf family bankrupt, and etc. Chuck needs to change, and he's failing to do so. So really, that sorry did nothing. That is why I am glad that for once Blair is taking a stand, and isn't all "BABY CHUCK, TAKE ME BACK!!!!" because that'd be pathetic.