I always get upset when someone explicitly agrees with me. Not because I don’t think what I’m saying, but because accepting my arguments as the Truth is an indication of non-critical thinking. Well yes, there is something as facts and a good starting point in any discussion would be, first to agree on facts. I’m talking about the argument (concept) development. I construct a concept (model) and try to improve on it. A healthy discussion is a useful tool to develop an idea and make it more adequate.
So, first, when someone agrees with me, that is to say: I don’t need this discussion — maybe I’m not interested or don’t feel at your level or simply having a more important matter on my mind. Second, I always hold in my mind more than one contradictory ideas concerning the matter, picking one of these and making it final, crashes my development state of mind (which is always self-contradictory at some level). Third, the recognition that “I’m right” gives me a strong feeling of recognizing my authority on the matter which limits my ability to speculate (always part of the developing process).