(1) Fanboys derive their sense of self-worth from their complete mastery of narrow and essentially meaningless topics, such as the arcane details of sci-fi and fantasy works.
(2) Identifying all the “alterations” gives them a chance to show off this mastery. The fact that NO ONE CARES is totally lost on them.
(3) One is marked almost as much by what one dislikes as what one likes, especially when one dislikes something that is popular or mainstream. For example, there are people who only will watch French films and proclaim their distaste for the allegedly inferior American films loudly and often to anyone who will listen. In their view, this marks them as people of taste, refinement and discernment who will not be bound by popular whims and fads. In other people’s view, they are pseudo-intellectual twits who lack any real capacity to enjoy life. As another example, I tend to drink imported beer and microbrews. To my mind, this marks me as someone with a little more taste (and income) than the masses that guzzle MGD. To others, I am an insufferable beer snob. But the point is, that by disliking something that is popular (such as the LOTR films), one marks oneself as an outsider, a contrarian, and a different kind of person. And, much as the fanboys might secretly want participate sufficiently in mainstream life to actually experience the kind of sex that involves participation of another human being, they also revel in their perceived status as outsiders. In their minds, they are elite and superior, and the disdain they arouse in most people is simply viewed as further proof that most people are stupider than they are.
(4) Fanboys always feel they could have done it better.
I’ll admit that at one point, I probably cared passionately about how warp drives actually worked and got offended when “Next Generation” changed those rules. And a whole bunch of other things. But then I grew up. And things like Elves at Helm’s Deep, Faramir being tempted by the Ring, no Tom Bombadil, and organic webshooters simply don’t bother me as much as they used to.