I only offer one possible explanation (there may be others) but this one certainly applies to me. By trade, I’m a Software Engineer and, as my project leader said “you’re being paid to be pedantic.” This is quite true: I have to be pedantic, because that thing that fucks up shit at some point in the future (i.e. the devil) is in the details. So I dissect, criticize and over-analysis stuff. You wouldn’t want it any other way though (just think about the software that runs in your set-top-box or medical equipment even). This need to critically analysis stuff spills into everything else I do though.
It’s not bad, it just means we see more “levels” to things. Take a flower, sure we can appreciate it’s beauty and why others find it beautiful, but we also appreciate it’s construction, the clever mechanisms of it’s survival and how it gets insects to carry it’s seeds and so on. Same with the movies we love, and because we love them, we take them apart, argue over what seem like trivia to others, recognise their flaws, etc. It doesn’t diminish our love for such things, but sure as heck pisses everyone else off. (Not that I have a problem appreciating something at a surface level. I love drawing and despite my amateur skills, I enjoy studying the surface and physical level nature of things when I draw).
My wife sometimes cuts me off when I correct our six year old daughter, because not only do I give the basic correction to her simply mistake, I try to address the underlying mistaken assumptions. I try to share with her my love of the details underneath.
It’s why, being a huge fan of Lord of the Rings (I’ve read all the books only three times so I’m not heavyweight) I didn’t like the movies but I accepted and enjoyed them for what they were.
Of course the side-effect is that with experience it makes you cynical. It’s why we hate marketing and “buzz” as it appears to be an attempt to gloss over and even give a different impression of (what we expect to be) the details. And to us, someone who is very enthusiastic about something can sometimes appear to be either a fool who hasn’t looked under the surface or a salesperson.
In conclusion, some of us geeks/nerds are pedants with good reason and hence we can appear to hate the things we love because we appear to be over-critical (but we are simply enjoying it in a different way).