it could be aspergers or high functioning autism...i have high functioning autism so i understand the unexcited stuff...they may be having a hard time understand social ques and reading facial expressions. I also have that same trouble with keeping on subject and i'll start a new topic randomly not knowing that everybody was focused on some different topic. If you give them weird looks or get angry at them they'll wonder what they did wrong and probably get upset. That's usually the case with me. And for people with aspergers or high functioning autism what's the point of focusing on something you don't understand. They could be getting lost in the conversation. People with aspergers and HFA have perseverations as well. Kind of what you would consider a hobby but they focus on that one thing and they probably know everything about that topic (or atleast alot) And if your friend is having trouble understanding simple things maybe try helping them understand and don't force it on them.
(oops...,rambled again. sorry if the above info didn't make sense. There is a book i would recommend if you want to know more about aspergers syndrome. It's by Tony Attwood a specialist in aspergers syndrome. The book is called " The Complete Guide to Asperger's Syndrome" And you have to remember they aren't broken just different)