No offense to anyone who are though, my best friend is a Genwunner and constantly complains his Black and White game sucks, even though he's at the 8th gym lol
This answers a big portion of your question. They still play and enjoy all the games, but they complain about them so that they can say "I liked it before it was cool" or "I liked it first." This gives them a feeling of superiority, which makes them feel good about themselves. They are most likely trying to make up for another part of their life in which they feel lacking.
It has nothing to do with actually believing gen 1 is the best in a lot of cases.
Certainly there are people who truly think gen 1 is the best. But they're not the ones following every Pokémon game and trolling and complaining on every generation's board.
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Because they are fans of Pokemon, just because they have a favorite gen shouldn't mean they hate Pokemon or all other gens.
People who fanboy over a single gen as the end all be all are either blatant trolls or ignorant of progress.
Being a fan of a gen more than the other? Common. We all have favorites, people like Charizard, but it is far from being the best Pokemon, and not even the best fire-starter.
I dont have a favorite. I have 4.5 . I do not like B&W. B&W2 I like though. So the first half of gen 5 is my least favorite. I still played B&W to try out the new pokemon and found a few I like. I even found two shinies of my favorites of Unova. Next gen I plan on using my Favorite from each region.
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