In essence, because he was just as selfish, dishonest, and manipulative as the people he professed to hate, but pretended otherwise because he believed that his relative poverty gave him some sort of moral high ground. read more
In addition in season 2 when he was involved with his teacher Rachel he said he cared about her and showed genuine concern towards her but since he was gossip girl at the time he was the one who nearly ruined her carreer by posting a tip Blair sent in. And so on. Theres tons of examples. read more
So the fact that Dan has played the part of the moral compass on Gossip Girl has pretty much always been a joke. From the beginning, he's been a hypocrite of sorts. And it really doesn't help that he's a writer. read more