Why did rivalries among european nations increases

The main two reasons why rivalries among European nations increased were 1) an increase in nationalism, and 2) and increase in militarism. These to occurrences were due largely to the spread of colonialism, which led powers to compete for land and resources. 

European nations were always hostile to a certain extent to each other in the past centuries, while not all were hostile to each other at the same time, there were skirmishes going on a lot of the times, together with political and economical schemes being present. 

However, this changed importantly with the rise of nationalism that favoured the right of self-determination and independence of each individual nation. This, coupled together with militarism, increased the rivalries that were already present in Europe due to historic reasons.