The guy I consider my best friend I think there is something wrong with him?

What it seem to me like, is that your friend is still hurt about what happen between his ex, its like he blame him self for what happen and is afraid that he may hurt someone or people may think of him the way she felt. And he does not want that so he has his guard up all the time, trying not to make anyone think of him that he changes with not even knowing. So the best thing I thing you could do for him is just be there for him and if you find that at times he not himself just give him time and he will come around.