Character.
Czechs generally take the word ‘friend’ and what it implies seriously. They would not call brief acquaintances ‘friends’.
So, you might say they a little more suspicious of strangers than other nationalities are, a little less open and a bit more reserved. But once you have got to know a Czech person and you two get along well and become ‘friends’ that does really mean something. Your friendship won’t be superficial and might last forever. It might take a while to achieve ‘friend status’ with a Czech and you might miss the wide-spread friendliness you encounter for instance in the USA. On the upside, Czechs don’t need to create the expression ‘best-est friend’.
Are Czechs gloomy? Some are, some aren’t. Truth to be told, the Czech Republic is not known for being extremely optimistic. Czech do not smile a lot, either, it seems. Amateur anthropologists like to describe this circumstance as a relic of Communist times, when the population did not have a lot to smile about. You probably won’t find bright smiles on shop clerks faces, may encounter short service. Depending on your own cultural background and character, you might find this rude or you might even find this Czech sincerity refreshing.
On a note, especially Prague has to deal with masses of tourists occupying the city all year round – many tourists do behave badly (just think of drunk stags!) attracted by the alleged cheapness of everything. Naturally, this gives all tourists a somewhat bad reputation maybe explaining why some innocent tourists encounter ‘suspicious, xenophobic Czechs’.