cognitive biases, to be precise; stupid or smart has little to do with it. The halo effect (from your perception of his IQ) and projection bias (expecting him to share your values) put in appearances. Also, looking at Charlie Munger's "Causes of Human Misjudgment," I see #15 liking distortion in there. All this is very human of you.
I recommend reading Robert Cialdini's Influence as a kind of vaccine. It took a very long time per page for me to read that book, because I had to spend so much time on subconscious processing between chapters.