The big reasons are this:
#1: America bashing, people get sick of it in the US. Heck, we bash ourselves enough without having some clueless person from another country get even more banal about it. Especially when they have the audacity to act like we're arrogant about what we do, missing all of the whining and self-criticism that goes on here.
See, I'm there for the games usually. But in general people from Europe want to go on about politics at least as an aside.
#2: Other countries want what we have. Face it the EU Was an attempt to reforge Europe into an economic powerhouse capable of replacing the US as the dominant global power/market. China (and a lot of Asia) want to rule the world both through economic and military means and spread their idealogy/culture. Needless to say our own interests (wanting to stay on top) conflict with these objectives, so even if the conflicts aren't "hot" they do lead to a lot of anger. You know like "Why does America stand in the way of us taking pieces of the pie they currently hold". Bitterness over us having things they don't and want.
Not to mention issues like The War On Terror might be "right" but when dealing with nations and countries that were pumping up their economies by trading with nations like Iraq illegally through things like the Oil For Food program, the affects of the conflict have been fairly pronounced, and of course people don't like it when it cuts into their livelihood. Not only have we lost our illegal trade monies, but we've also been put in a situation where it's more dangerous for us to try and trade with these people whom we never should have been trading with to begin with.
#3: There is also a racial issue, see when you get down to it Asians are VERY racist. Especially mainland Chinese (ie people from China, as opposed to people merely of the ethnicity) if you've dealt with many of them. There are whole issues of divine and spiritual destiny involved, and a kind of master race doctrine that would make Hitler blush. A lot of people are not aware of all of the details, but simply put you find a lot of Americans getting increasingly annoyed about it as awareness grows.
See, in the US you HAD a lot of Asiaphilles created by things like Anime, and Hong Kong action movies. With the East becoming more vocal in recent years people are beginning to run into more actual mainstream Japanese/Chinese people, as opposed to an unusually enlightened group of fanboys involved in fansubbing (ie relations born of mutal loves of science fiction, fantasy, and fandom). Nerds being pretty tolerant of other nerds, and probably one of the least racist groups out there, typically focused on their area of nerd-dom.
You deal with enough Chinese or Japanese people who treat you like a retarded house pet, and/or read some of their translated periodicals (even ones about games and such) and it's even more disturbing, and you start to see increasing irritation.
Then of course there is the entire political "your going down US, we're going to replace you, and then kick the cr@p out of you for years like you did to us" sentiments behind things too that make the Europeans merely annoying in comparison to the east-west hatred.
Add to this equasion all of the Ivory Tower Weeaboos who either don't see it, or maintain intentional ignorance, and you begin to see the issue. I mean at one point people just thought the Weeaboo/Wapanese thing was a bit wierd and maybe slightly annoying (but tolerable). In recent years it's become a target for mass hatred... well there is a connection. Listening to these guys after being insulted by racist asians tends to generate anger.
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Please note that this is merely my opinion/observations.
As far as Australians go, I play with a lot of them online. Truthfully while Aussies might not agree with everything the US is about, they are fairly laid back, and like a lot of people from the US are primarily there for the games. As such your not likely to get into many political arguements, unlike with a lot of Europeans, or Asians (many of whom go out of their way to try and belittle you).
Kiwis (New Zealanders) are also decent, but much more likely to go off on US-bashing political rants in gaming sessions.
Typically the thing is that you login to game, tired of all of the self-loathing crap in our media and re-invented history (evil white people! even more evil Americans! We've never done anything good). Then during your escapism some guy from the UK or Asia decides to start bashing America to the point where you feel the need to represent. Especially seeing as you've already had your brain saturated with this crud in your back yard. It gets to the point where you just want to play with other Americans so you can you know... play the games.
Probably generating a lot of TL

R sentiment here, and those that did read it probably won't agree. But let's just say one of the reasons why I am so militantly pro-American and Jingoistic, and go out of my way at certain times, and in certain forums to knock other countries that typically take cheap shots at the US (like Canada), is because I'm sick of all of this. If noone else has any national pride, I guess I'll project it and be just as obnoxious as everyone else... and hey, if 90% of the international user base already think Americans are arrogant jerks, it's not like actually being one to defend my own nation does any damage. It's just playing the same game back (and all exagerration in dialogue aside, I do tend to see the US as 'right' in the final equasion).
>>>----Therumancer--->