sometimes I find myself very frustrated with him because he does things that make no sense. It's like he tries to think way too hard about the easiest things and in the process he makes an easy task 10 x more difficult. This makes me think he is not too bright and I become irritated.
OP, my husband is extremely intelligent, genius level, Phd, works in research..the whole nine yards but I could have written the exact same sentence about my DH. Very, very smart people, in my experience, often lack common sense or make simple things way harder than they need to be. I often find myself thinking how can someone so smart do something so dumb.
DH is very nice, not egocentric, and not arrogant. While there are ego driven scientists, many of the smart ones are more introverted and lack the social sense to be arrogant if they wanted. I have found that people who desperately want to be smart or are on the edge of very smart tend to be more arrogant about showing everyone how smart they are insecure about themselves.