Ha, five centuries! We Slovenians have had 12. From 828 AD to 1991 AD.
But yes, us Slavs are a gloomy bunch. But only on the outside and only to strangers. Particularly Americans. Whenever I'm in the States I commonly get accused of being emotionless and of having a deadpan face. At the same time, I feel everybody I meet there is a clown and I always make sure to tune down their enthusiasm. If an American says something is super awesome and the best thing ever, it's probably kinda okayish.
Anyway, the interesting thing I've found comparing Slavs to Americans is that while Slavs are weary of strangers, they open up much sooner. You can be friends with Americans for months and years, but if you haven't gone to the same kindergarten together, you will never be a true friend. Whereas with Slavs, you could have met at a bar three hours ago, and already you're confessing your deepest darkest secrets to each other and are practically best friends.
Source: having lived in the States for some time
PS: to be fair, most Europeans I've met both in Europe and in the US share this immediately friends trait. So far none of the Americans I've met have, possibly because of The Big City effect.