I think what we all love about the hobby is the human-to-human interaction - the fact that not only are you moving around miniatures that you like and have lovingly put together to form an army, but that there is also a fellow nerd on the other side of the table who's just as weirdly enthusiastic about all this stuff as you are.
Yeah, but what if that other person is not as enthusiastic? What do to when your fellow gamer seems intent on ruining your fun for some reasons of his own?
In my own gaming group there's a Necron guy just like that - he would never smile or joke about anything, all he does is complain that "his Warriors are overpriced and useless, Wraiths are only decent and much worse than other stuff, Immortals are awful" etc etc, ad nauseam. What adds insult to injury is that I've had occasions where his Wraiths would massacre my entire unit in a single turn while he would still complain how bad they are.
Personally, I've found that a relatively good way of handling that is to just ignore most of such talk - while it may seem a bit rude, I think it works out best for everyone around. I mean, sometimes if you ignore a comment that Unit X is so awful for its point cost and concentrate on planning your next move, the other person may be discouraged enough to actually shut up. If you're good at this kind of stuff you could try to convince them otherwise (the "Hey man, your guys just killed half my army, guess they ain't so bad, eh?"), but that requires much too much patience than I'm willing to sacrifice (especially since it's so tedious after 4 or 5 games of same stuff over and over again). For me it's a bit insulting that I devote my time and effort in painting an army, thinking about how to make it the most efficient and then pitting my tactical acumen against my opponent only to have all those efforts destroyed because someone does not believe in his own army. I mean, hell, I don't think Guardians are as good a Troops choice as the Tactical Squad, but I celebrate every successful unit kill or passed Leadership test because they're the ordinary guys of my army - not to mention that I think it makes for much funnier games when a squad of Guardians refuses to die or even retreat despite being under heavy fire throughout half of the game, while at the same time an elite unit of Warp Spiders may decide to call it a day after one of them is consumed by the Warp when they use their generators.
What ways of dealing with negativity from other gamers do you have? What are the sources of negative hobby feelings around you (eg. personal problems, outdated Codexes, rules lawyering, whatever)? And do you think that this is an issue in the hobby, generally speaking? Thank God our community is not as toxic as the online ones (I'm looking at you, League of Legends), but there are rotten apples in every barrel.
Also, do you think that negative players can be convinced to change their ways? Most of the time you can just refuse to play some random dude in the hobby store, but when it's a member of your regular gaming group that's usually not really an option.