I think he just doesn't know how to be tactful- probably everyone at some point has had a friend who they didn't like everyhing about or who sometimes annoyed them, most people just know how to hide it so as not to be hurtful, but it just doesn't even cross his mind that he shouldn't say somethings- sort of like a little kid (like Dean said when he found out Cas told Sam about robo!Sam ( at least I think that was when he said it):-)