Why did nationalism increase rivalries between european nations

The main reason why nationalism increased rivalries between European nations is because it breaded an "us vs. them" mentality, in which every country began to feel separated from, and superior to, other nations. This eventually led to war. 

Do you have more info about it ? i doubt this is the answer because i don't have all the information but if trouble was brewing in a clinic maybe it was an argument as to which clinic was better or smth of that nature