Americans tend to smile more than Ukrainians, generally speaking. It's a cultural difference that can lead to Americans deciding that Ukrainians are dour, and (I'd guess) Ukrainians thinking Americans are worryingly chipper.
I don't know to Greeks Americans just look fun. :P And at times overexcited XD But then again we aren't famed for being quiet. XD
Quiet is not the image I have of Greeks. xD