my experience (as us-born/bred) is that this exists, but is more a personality trait than anything else. And some personality traits can co-exist among others with the same trait more easily, and that 'so excited' outward expression can often get amplified in a group, but even individuals can be like that. I don't see it as uniquely American, but... it may be somewhat here. By comparison, traveling in Russia, public displays of enthusiasm/positivity/emotion were far less common at all (although in small groups of friends they came out).