This is a theory I've been thinking and I want to share with you guys.
The superior veteran effect is an effect mostly present in fandoms, forums and professions although it could be in others group activities, basically the longer the person is exposed to certain activity the more prone he is to develop a superior complex and to dislike certain activity, to better explain myself I'm going to make a example of an anime forum site.
When a new member joins the site he is mostly positive and very active, he start commenting about his favorites animes, probably entry mainstream animes like naruto or bleach (sorry if this example is a little outdated) as time passes he start to become more experienced and he can distinguish the good anime from the bad ones he is now a "veteran".
A veteran can go for different paths, some becomes less and less active others try to be helpful and contribute with the community other just only talks with friends about random stuff and others develops this effect.
The thing with these ones is that their complaints make valid points but they are party poopers for example:
user: So the new season of X is released next week what are your predictions.
veteran user: Meh this show has become in total fan pandering so I dont expect anything good.