well i think he should at least trust serena and try to be trust her best friends and when it comes to serena, chuck is her brother and blair is her sister. but it wasn't until 2.5 that Dan even thought Chuck had any feelings because it was a lot easier for Dan to just see him as a villian. Chuck did start trusting Dan in 2.5, but Dan screwed himself over with his family. But then in 214 they were square again. Blair doesn't because of what happened with DS I think.