Why can I add comments to this particular user'a post?

So this user posted about how he was so jealous of justin Bieber and how he hated him boohoo crap like that. I don't like him either but I basically said that he was jealous of Bieber in his question and then he came Into one of my questions as non anonymous to make a whole point about how he wasn't jealous. So I tried to add a comment to his answer but I can't. Like on the bottom it's says "sign in to add a comment " even though I'm already signed in. Strange thing is I have no problem commenting other users answers? Canyou guys tell me what's going on?