There's also a difference between being positive and being forced to be positive.
Additionally it's got far, far broader implications. The positiveness in corporate America for example, this sense that if you want it bad enough you'll get it. The logical flip of this is that the people who don't get it just didn't want it enough, so it's their own fault. It's a really poisonous attitude because it over-empowers them in their own mind. Many, many Americans have this attitude and it really becomes a problem for them and the society. It shifts responsibility away from other people and onto you, making people shoulder the burdens for things which are way beyond their control. It isn't your fault, for example, that your boss has cut your wages or outsourced your job - it's his.
It is, essentially, the keystone of American social pacification. That may seem a little ridiculous at first glance, but just consider that the presence or absence of this attitude is one of the only two major differences between Europe and the USA - otherwise, we are shockingly similar in attitudes and culture. The other is the increased religiosity of the US, but arguably that's very tied to this same optimist-individualist dogma of American society. That aside, this optimist-individualist outlook leads people to personalise the social - witness the idea of recession-as-opportunity or unemployment-as-failure when in reality they're just insignificant people caught in the tide of events.
I think in Europe this attitude is rejected, though perhaps rejected isn't the best word because it doesn't convey the sense that there is no option of believing this without seeming a little ridiculous. This acceptance, though, this sense that you are insignificant actually almost empowers people. I think it's very interesting to note that outside of the black community in the US (who similarly tend to reject the optimist-individualist outlook) American politics occurs in a remarkable bubble where public outrage is only considered through the lens of what impact it will have in the coming elections.
Public outrage this side of the pond, however, has a tendency to turn into strikes, protests and riots.
I think that by accepting the social nature of many things we can keep ourselves for ourselves, that everything isn't a test or a judgement passed on ourselves but rather that things just happen to us. I'd hate to live in this world where everything that happens is a reflection of me or my attitude.